# Publications

2020
Mastichiadis A, Florou I, Kefala E, Boula SS, Petropoulou M. A roadmap to hadronic supercriticalities: a comprehensive study of the parameter space for high-energy astrophysical sources. [Internet]. 2020;495:2458 - 2474. WebsiteAbstract
Hadronic supercriticalities are radiative instabilities that appear when large amounts of energy are stored in relativistic protons. When the proton energy density exceeds some critical value, a runaway process is initiated resulting in the explosive transfer of the proton energy into electron-positron pairs and radiation. The runaway also leads to an increase of the radiative efficiency, namely the ratio of the photon luminosity to the injected proton luminosity. We perform a comprehensive study of the parameter space by investigating the onset of hadronic supercriticalities for a wide range of source parameters (I.e. magnetic field strengths of 1 G-100 kG and radii of 1011-1016 cm) and maximum proton Lorentz factors (103-109). We show that supercriticalities are possible for the whole range of source parameters related to compact astrophysical sources, like gamma-ray bursts and cores and jets of active galactic nuclei. We also provide an in-depth look at the physical mechanisms of hadronic supercriticalities and show that magnetized relativistic plasmas are excellent examples of non-linear dynamical systems in high-energy astrophysics.
Liodakis I, Petropoulou M. Proton Synchrotron Gamma-Rays and the Energy Crisis in Blazars. [Internet]. 2020;893:L20. WebsiteAbstract
The origin of high-energy emission in blazars jets (I.e., leptonic versus hadronic) has been a longstanding matter of debate. Here, we focus on one variant of hadronic models where proton synchrotron radiation accounts for the observed steady γ-ray blazar emission. Using analytical methods, we derive the minimum jet power ( ${P}_{j,\min }$ ) for the largest blazar sample analyzed to date (145 sources), taking into account uncertainties of observables and jet's physical parameters. We compare ${P}_{j,\min }$ against three characteristic energy estimators for accreting systems, I.e., the Eddington luminosity, the accretion disk luminosity, and the power of the Blandford-Znajek process, and find that ${P}_{j,\min }$ is about 2 orders of magnitude higher than all energetic estimators for the majority of our sample. The derived magnetic field strengths in the emission region require either large amplification of the jet's magnetic field (factor of 30) or place the γ-ray production site at sub-pc scales. The expected neutrino emission peaks at ∼0.1-10 EeV, with typical peak neutrino fluxes ∼10-4 times lower than the peak γ-ray fluxes. We conclude that if relativistic hadrons are present in blazar jets, they can only produce a radiatively subdominant component of the overall spectral energy distribution of the blazar's steady emission.
Liodakis I, Petropoulou M. VizieR Online Data Catalog: Proton synchrotron gamma-rays in blazars (Liodakis+, 2020). [Internet]. 2020:J/ApJ/893/L20. WebsiteAbstract
Parameter estimates for the sources in our sample. (1 data file).
Ji H, Alt A, Antiochos S, Baalrud S, Bale S, Bellan PM, Begelman M, Beresnyak A, Blackman EG, Brennan D, et al. Major Scientific Challenges and Opportunities in Understanding Magnetic Reconnection and Related Explosive Phenomena throughout the Universe. [Internet]. 2020:arXiv:2004.00079. WebsiteAbstract
This white paper summarizes major scientific challenges and opportunities in understanding magnetic reconnection and related explosive phenomena as a fundamental plasma process.
Petropoulou M, Murase K, Santander M, Buson S, Tohuvavohu A, Kawamuro T, Vasilopoulos G, Negoro H, Ueda Y, Siegel MH, et al. Multi-epoch Modeling of TXS 0506+056 and Implications for Long-term High-energy Neutrino Emission. [Internet]. 2020;891:115. WebsiteAbstract
The IceCube report of a ∼ 3.5σ excess of 13 ± 5 neutrino events in the direction of the blazar TXS 0506+056 in 2014-2015 and the 2017 detection of a high-energy neutrino event, IceCube-170922A, during a gamma-ray flare from the same blazar, have revived the interest in scenarios for neutrino production in blazars. We perform comprehensive analyses on the long-term electromagnetic emission of TXS 0506+056 using optical, X-ray, and gamma-ray data from the All-Sky Automated Survey for Supernovae, the Neil Gehrels Swift Observatory, Monitor of All-sky X-ray Image, and the Fermi Large Area Telescope. We also perform numerical modeling of the spectral energy distributions (SEDs) in four epochs prior to 2017 with contemporaneous gamma-ray and lower-energy (optical and/or X-ray) data. We find that the multi-epoch SEDs are consistent with a hybrid leptonic scenario, where the gamma-rays are produced in the blazar zone via external inverse Compton scattering of accelerated electrons, and high-energy neutrinos are produced via the photomeson production process of co-accelerated protons. The multi-epoch SEDs can be satisfactorily explained with the same jet parameters and variable external photon density and electron luminosity. Using the maximal neutrino flux derived for each epoch, we put an upper limit of ∼0.4-2 on the muon neutrino number in 10 years of IceCube observations. Our results are consistent with the IceCube-170922A detection, which can be explained as an upper fluctuation from the average neutrino rate expected from the source, but in strong tension with the 2014-2015 neutrino flare.
Christie IM, Petropoulou M, Sironi L, Giannios D. Interplasmoid Compton scattering and the Compton dominance of BL Lacs. [Internet]. 2020;492:549 - 555. WebsiteAbstract
Blazar emission models based on magnetic reconnection succeed in reproducing many observed spectral and temporal features, including the short-duration luminous flaring events. Plasmoids, a self-consistent by-product of the tearing instability in the reconnection layer, can be the main source of blazar emission. Kinetic simulations of relativistic reconnection have demonstrated that plasmoids are characterized by rough energy equipartition between their radiating particles and magnetic fields. This is the main reason behind the apparent shortcoming of plasmoid-dominated emission models to explain the observed Compton ratios of BL Lac objects. Here, we demonstrate that the radiative interactions among plasmoids, which have been neglected so far, can assist in alleviating this contradiction. We show that photons emitted by large, slow-moving plasmoids can be a potentially important source of soft photons to be then upscattered, via inverse Compton, by small fast-moving, neighbouring plasmoids. This interplasmoid Compton scattering process can naturally occur throughout the reconnection layer, imprinting itself as an increase in the observed Compton ratios from those short and luminous plasmoid-powered flares within BL Lac sources, while maintaining energy equipartition between radiating particles and magnetic fields.
Zhang TB, Petropoulou M, Murase K, Oikonomou F. A Neutral Beam Model for High-energy Neutrino Emission from the Blazar TXS 0506+056. [Internet]. 2020;889:118. WebsiteAbstract
The IceCube collaboration reported an ∼3.5σ excess of 13 ± 5 neutrino events in the direction of the blazar TXS 0506+056 during an ∼6 month period in 2014-2015, as well as the (∼3σ) detection of a high-energy muon neutrino during an electromagnetic flare in 2017. We explore the possibility that the 2014-2015 neutrino excess and the 2017 multimessenger flare are both explained in a common physical framework that relies on the emergence of a relativistic neutral beam in the blazar jet due to interactions of accelerated cosmic rays (CRs) with photons. We demonstrate that the neutral beam model provides an explanation for the 2014-2015 neutrino excess without violating X-ray and γ-ray constraints and yields results consistent with the detection of one high-energy neutrino during the 2017 flare. If both neutrino associations with TXS 05065+056 are real, our model requires that (I) the composition of accelerated CRs is light, with a ratio of helium nuclei to protons ≳5; (II) a luminous external photon field (∼1046 erg s-1) variable (on yearlong timescales) is present; and (III) the CR injection luminosity, as well as the properties of the dissipation region (I.e., Lorentz factor, magnetic field, and size), vary on yearlong timescales.
Yoshida K, Bailyn C, Cruz B, Urry C, Coppi P, Vasilopoulos G, Petropoulou M, Meyer M. Cross-Correlation Analysis between Gamma-ray and Optical/Infrared Variability for Bright Blazars Monitored in 2008-2017. In: Vol. 235. ; 2020. pp. 405.08. WebsiteAbstract
We present the results of cross-correlation analysis between the Fermi-LAT gamma-ray and SMARTS optical/infrared light curves of bright 8 blazars monitored in 2008-2017. For the temporal correlation analysis of unevenly sampled variability data, we use the Discrete Correlation Function (DCF), and created an empirical bootstrapping method to assess the significance of the DCF amplitude for each blazar. The DCFs between gamma-ray and optical/infrared light curves with one week binning time scale suggest that 6 of the 8 blazars show a significant peak at zero lag at or above 3 sigma level. That is consistent with the leptonic model in which optical/infrared photons are produced by synchrotron radiation of relativistic electrons and gamma rays are produced by inverse Compton scattering of ambient photons by the synchrotron-emitting electrons. However, the DCFs with one day binning time scale suggest that among 8 blazars, only one blazar — 3C 454.3 — still has a significant peak at zero lag. The other 7 blazars tend to show much smaller peaks than those with a weekly time bin. In addition, for a given blazar, strong changes of the DCFs from one epoch to the next are shown by the analyses of time periods of one or two years. These results complicate the simplest understanding of blazar emission mechanisms. We discuss possible physical explanations.
Ajello M, Inoue Y, Hartmann D, Bloser P, Finke J, Vestrand W, Prescod-Weinstein C, Oberlack U, Petropoulou M, The L, et al. The MeV Background with AMEGO. In: Vol. 235. ; 2020. pp. 372.16. WebsiteAbstract
The emission of our Universe is well characterized at most wavelengths, but a gap remains at MeV energies. This is an energy range where nuclear decays from type Ia supernovae (SNIa), emission from radio-loud and radio-quiet active galactic nuclei (AGN) and potentially dark matter interaction can each contribute to the MeV background. An all-sky MeV mission like AMEGO will allow us to measure the intensity and the angular fluctuations of the MeV background. This will allow us to constrain models of SNIa formation, the evolution of radio-loud and radio-quiet AGN, the growth of the most massive black holes and to constrain the cross-section for dark matter interaction.
Meyer M, Petropoulou M, Christie I. The observability of plasmoid-powered $\gamma$-ray flares with the Fermi Large Area Telescope. [Internet]. 2020:arXiv:2012.09944. WebsiteAbstract
The exact mechanism for the production of fast $\gamma$-ray variability in blazars remains debated. Magnetic reconnection, in which plasmoids filled with relativistic particles and magnetic fields are formed, is a viable candidate to explain the broadband electromagnetic spectrum and variability of these objects. Using state-of-the-art magnetic reconnection simulations, we generate realistic $\gamma$-ray light curves that would be observed with the Fermi Large Area Telescope (LAT). A comparison with observed $\gamma$-ray flares from flat spectrum radio quasars (FSRQs) reveals that magnetic reconnection events lead to comparable flux levels and variability patterns, in particular when the reconnection layer is slightly misaligned with the line of sight. Emission from fast plasmoids moving close to the line of sight could explain fast variability on the time scales of minutes for which evidence has been found in observations of FSRQs. Our results motivate improvements in existing radiative transfer simulations as well as dedicated searches for fast variability as evidence for magnetic reconnection events.
Murase K, Kimura SS, Zhang TB, Oikonomou F, Petropoulou M. High-energy Neutrino and Gamma-Ray Emission from Tidal Disruption Events. [Internet]. 2020;902:108. WebsiteAbstract
Tidal disruption events (TDE) have been considered as cosmic-ray and neutrino sources for a decade. We suggest two classes of new scenarios for high-energy multi-messenger emission from TDEs that do not have to harbor powerful jets. First, we investigate high-energy neutrino and gamma-ray production in the core region of a supermassive black hole. In particular, we show that ∼1-100 TeV neutrinos and MeV gamma rays can efficiently be produced in hot coronae around an accretion disk. We also study the consequences of particle acceleration in radiatively inefficient accretion flows (RIAFs). Second, we consider possible cosmic-ray acceleration by sub-relativistic disk-driven winds or interactions between tidal streams, and show that subsequent hadronuclear and photohadronic interactions inside the TDE debris lead to GeV-PeV neutrinos and sub-GeV cascade gamma rays. We demonstrate that these models should be accompanied by soft gamma rays or hard X-rays as well as optical/UV emission, which can be used for future observational tests. Although this work aims to present models of non-jetted high-energy emission, we discuss the implications of the TDE AT2019dsg that might coincide with the high-energy neutrino IceCube-191001A, by considering the corona, RIAF, hidden sub-relativistic wind, and hidden jet models. It is not yet possible to be conclusive about their physical association and the expected number of neutrinos is typically much less than unity. We find that the most optimistic cases of the corona and hidden wind models could be consistent with the observation of IceCube-191001A, whereas jet models are unlikely to explain the multi-messenger observations.
Mastichiadis A, Petropoulou M. Hadronic X-ray Flares from Blazars. [Internet]. 2020:arXiv:2009.12158. WebsiteAbstract
The detection of a high-energy neutrino from the flaring blazar TXS 0506+056 and the subsequent discovery of a neutrino excess from the same direction have strengthened the hypothesis that blazars are cosmic neutrino sources. The lack, however, of $\gamma$-ray flaring activity during the latter period challenges the standard scenario of correlated $\gamma$-ray and high-energy neutrino emission in blazars. We propose instead that TeV-PeV neutrinos are produced in coincidence with X-ray flares that are powered by proton synchrotron radiation. In this case, neutrinos are produced by photomeson interactions of protons with their own synchrotron radiation, while MeV to GeV $\gamma$-rays are the result of synchrotron-dominated electromagnetic cascades developed in the source. Using a time-dependent approach, we find that this "pure hadronic flaring" hypothesis has several interesting consequences. The X-ray flux is a good proxy for the all-flavor neutrino flux, while certain neutrino-rich X-ray flares may be dark in GeV-TeV $\gamma$-rays. Lastly, hadronic X-ray flares are accompanied by an equally bright MeV component that is detectable by proposed missions like e-ASTROGAM and AMEGO. We then applied this scenario to the extreme blazar 3HSP J095507.9+355101 that has been associated with IceCube-200107A while undergoing an X-ray flare. We showed that the number of muon and antimuon neutrinos above 100 TeV during hadronic flares can be up to $\sim3-10$ times higher than the expected number in standard leptohadronic models. Still, frequent hadronic flaring activity is necessary for explaining the detected neutrino event IceCube-200107A.
Ji H, Karpen J, Alt A, Antiochos S, Baalrud S, Bale S, Bellan PM, Begelman M, Beresnyak A, Bhattacharjee A, et al. Major Scientific Challenges and Opportunities in Understanding Magnetic Reconnection and Related Explosive Phenomena in Solar and Heliospheric Plasmas. [Internet]. 2020:arXiv:2009.08779. WebsiteAbstract
Magnetic reconnection underlies many explosive phenomena in the heliosphere and in laboratory plasmas. The new research capabilities in theory/simulations, observations, and laboratory experiments provide the opportunity to solve the grand scientific challenges summarized in this whitepaper. Success will require enhanced and sustained investments from relevant funding agencies, increased interagency/international partnerships, and close collaborations of the solar, heliospheric, and laboratory plasma communities. These investments will deliver transformative progress in understanding magnetic reconnection and related explosive phenomena including space weather events.
Petropoulou M, Oikonomou F, Mastichiadis A, Murase K, Padovani P, Vasilopoulos G, Giommi P. Comprehensive Multimessenger Modeling of the Extreme Blazar 3HSP J095507.9+355101 and Predictions for IceCube. [Internet]. 2020;899:113. WebsiteAbstract
3HSP J095507.9+355101 is an extreme blazar that has been possibly associated with a high-energy neutrino (IceCube-200107A) detected 1 day before the blazar was found to undergo a hard X-ray flare. We perform a comprehensive study of the predicted multimessenger emission from 3HSP J095507.9+355101 during its recent X-ray flare, but also in the long term. We focus on one-zone leptohadronic models, but we also explore alternative scenarios: (i) a blazar-core model, which considers neutrino production in the inner jet, close to the supermassive black hole; (ii) a hidden external-photon model, which considers neutrino production in the jet through interactions with photons from a weak broad line region; (iii) a proton-synchrotron model, where high-energy protons in the jet produce γ-rays via synchrotron; and (iv) an intergalactic cascade scenario, where neutrinos are produced in the intergalactic medium by interactions of a high-energy cosmic-ray beam escaping the jet. The Poisson probability to detect a single muon neutrino in 10 years from 3HSP J095507.9+355101 with the real-time IceCube alert analysis is ∼1% (3%) for the most optimistic one-zone leptohadronic model (the multi-zone blazar-core model). Meanwhile, detection of a single neutrino during the 44-day-long high X-ray flux-state period following the neutrino detection is 0.06%, according to our most optimistic leptohadronic model. The most promising scenarios for neutrino production also predict strong intrasource γ-ray attenuation above ∼100 GeV. If the association is real, then IceCube-Gen2 and other future detectors should be able to provide additional evidence for neutrino production in 3HSP J095507.9+355101 and other extreme blazars.
Petropoulou M, Beniamini P, Vasilopoulos G, Giannios D, Barniol Duran R. Deciphering the properties of the central engine in GRB collapsars. [Internet]. 2020;496:2910 - 2921. WebsiteAbstract
The central engine in long gamma-ray bursts (GRBs) is thought to be a compact object produced by the core collapse of massive stars, but its exact nature (black hole or millisecond magnetar) is still debatable. Although the central engine of GRB collapsars is hidden to direct observation, its properties may be imprinted on the accompanying electromagnetic signals. We aim to decipher the generic properties of central engines that are consistent with prompt observations of long GRBs detected by the Burst Alert Telescope (BAT) on board the Neil Gehrels Swift Observatory. Adopting a generic model for the central engine, in which the engine power and activity time-scale are independent of each other, we perform Monte Carlo simulations of long GRBs produced by jets that successfully breakout from the star. Our simulations consider the dependence of the jet breakout time-scale on the engine luminosity and the effects of the detector's flux threshold. The two-dimensional (2D) distribution of simulated detectable bursts in the gamma-ray luminosity versus gamma-ray duration plane is consistent with the observed one for a range of parameter values describing the central engine. The intrinsic 2D distribution of simulated collapsar GRBs peaks at lower gamma-ray luminosities and longer durations than the observed one, a prediction that can be tested in the future with more sensitive detectors. Black hole accretors, whose power and activity time are set by the large-scale magnetic flux through the progenitor star and stellar structure, respectively, are compatible with the properties of the central engine inferred by our model.
Zhang H, Christie IM, Petropoulou M, Rueda-Becerril JM, Giannios D. Inverse Compton signatures of gamma-ray burst afterglows. [Internet]. 2020;496:974 - 986. WebsiteAbstract
The afterglow emission from gamma-ray bursts (GRBs) is believed to originate from a relativistic blast wave driven into the circumburst medium. Although the afterglow emission from radio up to X-ray frequencies is thought to originate from synchrotron radiation emitted by relativistic, non-thermal electrons accelerated by the blast wave, the origin of the emission at high energies (HE; ≳GeV) remains uncertain. The recent detection of sub-TeV emission from GRB 190114C by the Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) raises further debate on what powers the very high energy (VHE; ≳300 GeV) emission. Here, we explore the inverse Compton scenario as a candidate for the HE and VHE emissions, considering two sources of seed photons for scattering: synchrotron photons from the blast wave (synchrotron self-Compton or SSC) and isotropic photon fields external to the blast wave (external Compton). For each case, we compute the multiwavelength afterglow spectra and light curves. We find that SSC will dominate particle cooling and the GeV emission, unless a dense ambient infrared photon field, typical of star-forming regions, is present. Additionally, considering the extragalactic background light attenuation, we discuss the detectability of VHE afterglows by existing and future gamma-ray instruments for a wide range of model parameters. Studying GRB 190114C, we find that its afterglow emission in the Fermi-Large Area Telescope (LAT) band is synchrotron dominated. The late-time Fermi-LAT measurement (I.e. t ∼ 104 s), and the MAGIC observation also set an upper limit on the energy density of a putative external infrared photon field (I.e. ${\lesssim} 3\times 10^{-9}\, {\rm erg\, cm^{-3}}$ ), making the inverse Compton dominant in the sub-TeV energies.
Beniamini P, Duran RB, Petropoulou M, Giannios D. Ready, Set, Launch: Time Interval between a Binary Neutron Star Merger and Short Gamma-Ray Burst Jet Formation. [Internet]. 2020;895:L33. WebsiteAbstract
The joint detection of GW170817/GRB 170817 confirmed the long-standing theory that binary neutron star mergers produce short gamma-ray burst (sGRB) jets that can successfully break out of the surrounding ejecta. At the same time, the association with a kilonova provided unprecedented information regarding the physical properties (such as masses and velocities) of the different ejecta constituents. Combining this knowledge with the observed luminosities and durations of cosmological sGRBs detected by the Burst Alert Telescope onboard the Neil Gehrels Swift Observatory, we revisit the breakout conditions of sGRB jets. Assuming self-collimation of sGRB jets does not play a critical role, we find that the time interval between the binary merger and the launch of a typical sGRB jet is $\lesssim 0.1\,{\rm{s}}$ . We also show that for a fraction of at least $\sim 30 \%$ of sGRBs, the usually adopted assumption of static ejecta is inconsistent with observations, even if the polar ejecta mass is an order of magnitude smaller than that in GRB 170817. Our results disfavor magnetar central engines for powering cosmological sGRBs, limit the amount of energy deposited in the cocoon prior to breakout, and suggest that the observed delay of ∼1.7 s in GW170817/GRB 170817 between the gravitational wave and gamma-ray signals is likely dominated by the propagation time of the jet to the gamma-ray production site.
Hakobyan H, Petropoulou M, Spitkovsky A, Sironi L. Secondary Energization in Compressing Plasmoids during Magnetic Reconnection. [Internet]. 2020:arXiv:2006.12530. WebsiteAbstract
Plasmoids -- magnetized quasi-circular structures formed self-consistently in reconnecting current sheets -- were previously considered to be the graveyards of energetic particles. In this paper, we demonstrate the important role of plasmoids in shaping the particle energy spectrum in relativistic reconnection (i.e., with upstream magnetization $\sigma_{\rm up} \gg 1$). Using two dimensional particle-in-cell simulations in pair plasmas with $\sigma_{\rm up}=10$ and $100$, we study a secondary particle energization process that takes place inside compressing plasmoids. We demonstrate that plasmoids grow in time, while their interiors compress, amplifying the internal magnetic field. The magnetic field felt by particles injected in an isolated plasmoid increases linearly with time, which leads to particle energization as a result of magnetic moment conservation. For particles injected with a power-law distribution function, this energization process acts in such a way that the shape of the injected power law is conserved, while producing an additional non-thermal tail $f(E)\propto E^{-3}$ at higher energies followed by an exponential cutoff. The cutoff energy, which increases with time as $E_{\rm cut}\propto\sqrt{t}$, can greatly exceed $\sigma_{\rm up} m_e c^2$. We analytically predict the secondary acceleration timescale and the shape of the emerging particle energy spectrum, which can be of major importance in certain astrophysical systems, such as blazar jets.
Vasilopoulos G, Ray P, Koliopanos F, Petropoulou M, Haberl F, Lander S. Understanding spin-evolution and super-orbital modulation of Ultraluminous X-ray pulsars. In: Vol. 236. ; 2020. pp. 133.01. WebsiteAbstract
Ultra luminous X-ray pulsars (ULXP) are fascinating objects, whose X-ray emission greatly exceeds the Eddington limit for a solar mass object. Given the coherent pulsations we now know that these systems host accreting magnetized Neutron Stars (NS), thus challenging our understanding of accretion theory. Moreover several of these systems show super-orbital variability where the observed flux change by factor more than 10. Key questions about the nature of these systems are; is there is beaming involved that enhances the derived isotropic Luminosity? What is the magnetic field of the NS in ULXPs and how this compares to the typical X-ray pulsars? and what is the nature of the super-orbital modulation? The study of individual ULXPS can help us answer these key questions. Here I will present observational constrains on the properties of NGC 300 ULX1. Through a year long X-ray monitoring we discovered that even when the X-ray flux of the system decreased by a factor of 20 the spin-up of the NS continues at a constant rate denoting constant mass accretion onto the NS. Moreover, I will discuss the changes in the observed flux in the context of a precessing disc and outflows. In addition, I will discuss the properties of newly confirmed ULXP M51 ULX7, I will show that outflows are not a necessary requirement to account for super orbital variability, and will discuss alternative mechanisms.
2019
Petropoulou M, Murase K, Brindley Fox D, Kawai N. Neutrinos from hadronic cascades: the case of the 2014-15 neutrino flare from TXS 0506+056. In: Vol. 17. ; 2019. pp. 106.35. WebsiteAbstract
Gamma-ray flares from blazars have been suggested as ideal periods for the detection of high-energy neutrinos. Indeed, the first ∼3σ high-energy neutrino source association was based on the detection of a single neutrino (IC-170922) coincident with the flaring blazar TXS 0506+056. A follow-up analysis of IceCube archival data revealed a past "neutrino flare" (13 +/- 5 events within ~ 6 months) from the direction of TXS 0506+056 which, however, was not accompanied by any electromagnetic flare. Here, we investigate whether leptohadronic models of blazar emission can explain the 2014-15 neutrino flare without violating existing electromagnetic observations. To do so, we perform a wide scan of the available parameter space and numerically compute the neutrino and electromagnetic emission of the hadronic cascade for ~ 50 parameter sets. We explore both synchrotron-supported and Compton-supported electromagnetic cascades in the linear and non-linear regimes. We compare our model predictions against publicly available data from IceCube and Fermi-LAT and the X-ray upper limits we derived by analyzing archival BAT and MAXI data. We find no model that can simultaneously explain the neutrino flare and satisfy all electromagnetic constraints, thus implying the presence of more than one emitting regions in TXS 0506+056.
Petropoulou M, Sironi L, Spitkovsky A, Giannios D. Relativistic reconnection in pair-proton plasmas and application to AGN jets. In: Vol. 17. ; 2019. pp. 106.25. WebsiteAbstract
Magnetic field dissipation via reconnection is a promising process for explaining the non-thermal signatures from a variety of relativistic astrophysical outflows, such as pulsar wind nebulae (PWNe) and jets of active galactic nuclei (AGN). In most relativistic astrophysical outflows reconnection proceeds in the so-called relativistic regime in which the Alfven velocity of the plasma approaches the speed of light. In contrast to PWNe, where the outflow is composed of relativistic pairs, in AGN jets the composition of the plasma is largely unknown. Our goal is to study the general properties of relativistic reconnection in the unexplored regime of plasmas with mixed particle composition. We focus on pair-proton plasmas, as they bridge the gap between the pair plasma and electron-proton plasma cases that have been extensively studied in the past. We perform a suite of 2D PIC simulations using the realistic proton-to-electron mass ratio (mi/me=1836) while varying three physical parameters, namely the plasma magnetization, the plasma temperature, and the pair multiplicity. We study, for the first time, the energy distributions of accelerated particles, the inflows and outflows of plasma in the reconnection region, and the energy partition between pairs, protons, and magnetic fields, as a function of the pair multiplicity in the regime where protons dominate the rest mass energy of the plasma. We finally discuss our results in the context of non-thermal emission from AGN jets.
Ray PS, Arzoumanian Z, Ballantyne D, Bozzo E, Brandt S, Brenneman L, Chakrabarty D, Christophersen M, DeRosa A, Feroci M, et al. STROBE-X: X-ray Timing and Spectroscopy on Dynamical Timescales from Microseconds to Years. [Internet]. 2019:arXiv:1903.03035. WebsiteAbstract
We present the Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays (STROBE-X), a probe-class mission concept selected for study by NASA. It combines huge collecting area, high throughput, broad energy coverage, and excellent spectral and temporal resolution in a single facility. STROBE-X offers an enormous increase in sensitivity for X-ray spectral timing, extending these techniques to extragalactic targets for the first time. It is also an agile mission capable of rapid response to transient events, making it an essential X-ray partner facility in the era of time-domain, multi-wavelength, and multi-messenger astronomy. Optimized for study of the most extreme conditions found in the Universe, its key science objectives include: (1) Robustly measuring mass and spin and mapping inner accretion flows across the black hole mass spectrum, from compact stars to intermediate-mass objects to active galactic nuclei. (2) Mapping out the full mass-radius relation of neutron stars using an ensemble of nearly two dozen rotation-powered pulsars and accreting neutron stars, and hence measuring the equation of state for ultradense matter over a much wider range of densities than explored by NICER. (3) Identifying and studying X-ray counterparts (in the post-Swift era) for multiwavelength and multi-messenger transients in the dynamic sky through cross-correlation with gravitational wave interferometers, neutrino observatories, and high-cadence time-domain surveys in other electromagnetic bands. (4) Continuously surveying the dynamic X-ray sky with a large duty cycle and high time resolution to characterize the behavior of X-ray sources over an unprecedentedly vast range of time scales. STROBE-X's formidable capabilities will also enable a broad portfolio of additional science.
Paliya VS, Ajello M, Marcotulli L, Tomsick J, Perkins JS, Prandini E, D'Ammando F, De Angelis A, Thompson D, Li H, et al. Supermassive black holes at high redshifts. [Internet]. 2019:arXiv:1903.06106. WebsiteAbstract
MeV blazars are the most luminous persistent sources in the Universe and emit most of their energy in the MeV band. These objects display very large jet powers and accretion luminosities and are known to host black holes with a mass often exceeding $10^9 M_{\odot}$. An MeV survey, performed by a new generation MeV telescope which will bridge the entire energy and sensitivity gap between the current generation of hard X-ray and gamma-ray instruments, will detect $>$1000 MeV blazars up to a redshift of $z=5-6$. Here we show that this would allow us: 1) to probe the formation and growth mechanisms of supermassive black holes at high redshifts, 2) to pinpoint the location of the emission region in powerful blazars, 3) to determine how accretion and black hole spin interplay to power the jet.
Beniamini P, Petropoulou M, Duran RB, Giannios D. A lesson from GW170817: most neutron star mergers result in tightly collimated successful GRB jets. [Internet]. 2019;483:840 - 851. WebsiteAbstract
The joint detection of gravitational waves (GWs) and γ-rays from a binary neutron star (NS) merger provided a unique view of off-axis gamma-ray bursts (GRBs) and an independent measurement of the NS merger rate. Comparing the observations of GRB170817 with those of the regular population of short GRBs (sGRBs), we show that an order unity fraction of NS mergers result in sGRB jets that breakout of the surrounding ejecta. We argue that the luminosity function of sGRBs, peaking at {≈ } 2× 10^{52} erg s^{-1}, is likely an intrinsic property of the sGRB central engine and that sGRB jets are typically narrow with opening angles θ0 ≈ 0.1. We perform Monte Carlo simulations to examine models for the structure and efficiency of the prompt emission in off-axis sGRBs. We find that only a small fraction (∼0.01-0.1) of NS mergers detectable by LIGO/VIRGO in GWs is expected to be also detected in prompt γ-rays and that GW170817-like events are very rare. For an NS merger rate of ∼1500 Gpc-3 yr-1, as inferred from GW170817, we expect within the next decade up to ∼12 joint detections with off-axis GRBs for structured-jet models and just ∼1 for quasi-spherical cocoon models where γ-rays are the result of shock breakout. Given several joint detections and the rates of their discoveries, the different structure models can be distinguished. In addition the existence of a cocoon with a reservoir of thermal energy may be observed directly in the ultraviolet, given a sufficiently rapid localization of the GW source.
Yoshida K, Bailyn C, Cruz B, Urry CM, Coppi P, Vasilopoulous G, Petropoulou M. Correlated Infrared-Gamma-ray Variability in Bright, Well-Monitored Blazars 2008-2017. In: Vol. 233. ; 2019. pp. 454.10. WebsiteAbstract
We present cross correlations of the J-band SMARTS light curves and Fermi gamma-ray light curves for 8 bright blazars that have been monitored extensively on sub-weekly time scales over the past decade. Because of the uneven temporal sampling, we use the Discrete Correlation Function (DCF) and we create an empirical boot-strapping method to assess the significance of the DCF amplitude for each blazar. Our results are perhaps surprising. Early on in the Fermi mission, the flaring blazar 3C454.3 showed zero lag between optical and gamma-ray or infrared and gamma-ray fluxes, which Bonning et al. (2012) suggested was consistent with the gamma rays being produced by inverse Compton scattering of ambient photons by synchrotron-emitting electrons. However, of the 8 blazars we examine, only one - 3C454.3 - shows a significant peak at zero lag. The other seven show no significant peak at zero lag. Some blazars show broad peaks at lags of 10s of days, at or just below 3 sigma significance. In addition, analyses of time periods of a year or two only, for a given blazar, show strong changes from one epoch to the next. These results complicate our understanding of blazar emission mechanisms. Possible physical explanations are discussed.
Christie IM, Petropoulou M, Sironi L, Giannios D. Radiative signatures of plasmoid-dominated reconnection in blazar jets. [Internet]. 2019;482:65 - 82. WebsiteAbstract
The multiwavelength spectral and temporal variability observed in blazars set tight constraints on current theoretical emission models. Here, we investigate the relativistic magnetic reconnection process as a source of blazar emission in which quasi-spherical plasmoids, containing relativistic particles and magnetic fields, are associated with the emission sites in blazar jets. By coupling recent two-dimensional particle-in-cell simulations of relativistic reconnection with a time-dependent radiative transfer code, we compute the non-thermal emission from a chain of plasmoids formed during a reconnection event. The derived photon spectra display characteristic features observed in both BL Lac sources and flat spectrum radio quasars, with the distinction made by varying the strength of the external photon fields, the jet magnetization, and the number of pairs per proton contained within. Light curves produced from reconnection events are composed of many fast and powerful flares that appear on excess of a slower evolving envelope produced by the cumulative emission of medium-sized plasmoids. The observed variability is highly dependent upon the orientation of the reconnection layer with respect to the blazar jet axis and to the observer. Our model provides a physically motivated framework for explaining the multitime-scale blazar variability across the entire electromagnetic spectrum.
Guiriec S, Tomsick J, Hartmann D, Brandt T, Ajello M, De Angelis A, Bissaldi E, Rani B, Wadiasingh Z, Timmes F, et al. Gamma-Ray Science in the 2020s. [Internet]. 2019;2020:398. WebsiteAbstract
With very large fields of view, high-cadence sampling, high angular, and spectral resolutions, and polarization capabilities, the new generation gamma- ray instruments are ready to address the most pressing science questions of the next decades, and they are essential for the time-domain multi-messenger era.
Rani B, Zhang H, Hunter SD, Kislat F, Böttcher M, McEnery JE, Thompson DJ, Giannios D, Guo F, Li H, et al. High-Energy Polarimetry - a new window to probe extreme physics in AGN jets. [Internet]. 2019;51:348. WebsiteAbstract
We discuss the scientific potentials of gamma-ray polarimetry including the theoretical implications, and observational technology advances being made. We explore the primary scientific opportunities and wealth of information expected from synergy of multi-wavelength polarimetry that will be brought to multi-messenger astronomy.
Ji H, Alt A, Antiochos S, Baalrud S, Bale S, Bellan PM, Begelman M, Beresnyak A, Blackman EG, Brennan D, et al. Major Scientific Challenges and Opportunities in Understanding Magnetic Reconnection and Related Explosive Phenomena throughout the Universe. [Internet]. 2019;51:5. WebsiteAbstract
This is a group white paper of 100 authors (each with explicit permission via email) from 51 institutions on the topic of magnetic reconnection which is relevant to 6 thematic areas. Grand challenges and research opportunities are described in observations, numerical modeling and laboratory experiments in the upcoming decade.
Ajello M, Inoue Y, Bloser P, Vestrand WT, Hartmann D, Finke J, Wadiasingh Z, Prescod-Weinstein C, Oberlack U, Longo F, et al. The MeV Background. [Internet]. 2019;51:290. WebsiteAbstract
The emission of our Universe at MeV energies is unknown. New measurements of the intensity and the angular fluctuations of the MeV background will allow us to constrain models of SNIa formation, the evolution of radio-loud and radio-quiet AGN, the growth of the most massive black holes and to constrain the cross-section for dark matter interaction
Rani B, Petropoulou M, Zhang H, D'Ammando F, Finke J, Baring M, Boettcher M, Dimitrakoudis S, Gan Z, Giannios D, et al. Multi-Physics of AGN Jets in the Multi-Messenger Era. [Internet]. 2019;51:92. WebsiteAbstract
This new era of multi-messenger astronomy, which will mature in the next decade, offers us the unprecedented opportunity to combine more than one messenger to solve some long-standing puzzles of AGN jet physics. We advocate the support to future instruments with large effective areas, excellent timing resolution, and wide fields of view.
Ojha R, Zhang H, Kadler M, Neilson NK, Kreter M, McEnery J, Buson S, Caputo R, Coppi P, D'Ammando F, et al. Neutrinos, Cosmic Rays, and the MeV Band. [Internet]. 2019;51:431. WebsiteAbstract
The Mev band holds the key to answering three astrophysical questions: the sites where cosmic Rays are produced and accelerated, the origins of high- energy neutrinos, and the physical mechanisms producing the high energy gamma- ray emission from blazars. Theoretical and experimental capabilities needed in the next decade are discussed.
Ajello M, Paliya V, Marcotulli L, Perkins JS, Prandini E, D'Ammando F, De Angelis A, Thompson D, Li H, Dominguez A, et al. Supermassive black holes at high redshifts. [Internet]. 2019;51:289. WebsiteAbstract
MeV blazars are the most luminous sources in the Universe and host supermassive black holes. An MeV survey will detect >1000 of them up to z > 5. This would allow us: to probe the formation and growth of massive black holes at high z; to pinpoint the emission region location in blazars; to determine the interplay of accretion and black hole spin.
Santander M, Buson S, Fang K, Keivani A, Maccarone T, Murase K, Petropoulou M, Taboada I, Whitehorn N. A Unique Messenger to Probe Active Galactic Nuclei: High-Energy Neutrinos. [Internet]. 2019;51:228. WebsiteAbstract
We advocate for a multi-messenger approach that combines high-energy neutrino and broad multi-wavelength electromagnetic observations to study AGN during the coming decade. The unique capabilities of these joint observations promise to solve several long-standing issues in our understanding of AGN as powerful cosmic accelerators.
Yoshida K, Bailyn C, Cruz B, Urry MC, Coppi P, Vasilopoulous G, Petropoulou M. Correlations between Optical/Infrared and Gamma-ray Variability for Bright Blazars Monitored in 2008-2017. In: Vol. 17. ; 2019. pp. 106.71. WebsiteAbstract
We report the results of cross correlations of the SMARTS optical/infrared and Fermi-LAT gamma-ray light curves for 8 bright blazars that have been monitored with 1 day resolution over the past decade. For the temporal correlation analysis of unevenly sampled variability data, we use the Discrete Correlation Function (DCF), creating an empirical bootstrapping method to assess the significance of the DCF amplitude for each blazar. Our results are perhaps surprising. Early on in the Fermi mission, the brightest gamma-ray blazar 3C 454.3 showed zero lag between optical/infrared and gamma-ray fluxes as reported by Bonning et al. (2012), which was consistent with the leptonic model that optical/infrared photons are produced by synchrotron radiation of relativistic electrons and gamma rays are produced by inverse Compton scattering of ambient photons by the synchrotron-emitting electrons. However, among the 8 blazars, only one blazar - 3C 454.3 - shows a significant peak at zero lag, and the other 7 blazars show no significant peak at zero lag. Some blazars show broad peaks at tens of days of lags at or just below 3 sigma significance. In addition, for a given blazar, strong changes of the DCFs from one epoch to the next are shown by the analyses of time periods of one or two year. These results make it complicated to understand blazar emission mechanisms. We discuss possible physical explanations.
Padovani P, Oikonomou F, Petropoulou M, Giommi P, Resconi E. TXS 0506+056, the first cosmic neutrino source, is not a BL Lac. [Internet]. 2019;484:L104 - L108. WebsiteAbstract
We present evidence that TXS 0506+056, the first plausible non-stellar neutrino source, despite appearances, is not a blazar of the BL Lac type but is instead a masquerading BL Lac, i.e. intrinsically a flat-spectrum radio quasar with hidden broad lines and a standard accretion disc. This reclassification is based on: (1) its radio and O II luminosities; (2) its emission line ratios; (3) its Eddington ratio. We also point out that the synchrotron peak frequency of TXS 0506+056 is more than two orders of magnitude larger than expected by the so-called blazar sequence', a scenario which has been assumed by some theoretical models predicting neutrino (and cosmic ray) emission from blazars. Finally, we comment on the theoretical implications this reclassification has on the location of the γ-ray emitting region and our understanding of neutrino emission in blazars.
Oikonomou F, Murase K, Petropoulou M. High-Energy Neutrinos from Blazar Flares and Implications of TXS 0506+056. In: Vol. 210. ; 2019. pp. 03006. WebsiteAbstract
Motivated by the observation of a > 290 TeV muon neutrino by IceCube, coincident with a 6 month-long γ-ray flare of the blazar TXS 0506+056, and an archival search which revealed 13 ± 5 further, lower-energy neutrinos in the direction of the source in 2014-2015 we discuss the likely contribution of blazars to the diffuse high-energy neutrino intensity, the implications for neutrino emission from TXS 0506+056 based on multi-wavelength observations of the source, and a multi-zone model that allows for sufficient neutrino emission so as to reconcile the multi-wavelength cascade constraints with the neutrino emission seen by IceCube in the direction of TXS 0506+056.
Vasilopoulos G, Petropoulou M, Koliopanos F, Ray PS, Bailyn CB, Haberl F, Gendreau K. NGC 300 ULX1: spin evolution, super-Eddington accretion, and outflows. [Internet]. 2019;488:5225 - 5231. WebsiteAbstract
NGC 300 ULX1 is an ultraluminous X-ray pulsar, showing an unprecedented spin evolution, from about 126 s to less than 20 s in only 4 yr, consistent with steady mass accretion rate. Following its discovery we have been monitoring the system with Swift and NICER to further study its properties. We found that even though the observed flux of the system dropped by a factor of ≳20, the spin-up rate remained almost constant. A possible explanation is that the decrease in the observed flux is a result of increased absorption of obscuring material due to outflows or a precessing accretion disc.
McEnery J, van der Horst A, Dominguez A, Moiseev A, Marcowith A, Harding A, Lien A, Giuliani A, Inglis A, Ansoldi S, et al. All-sky Medium Energy Gamma-ray Observatory: Exploring the Extreme Multimessenger Universe. In: Vol. 51. ; 2019. pp. 245. WebsiteAbstract
The All-sky Medium Energy Gamma-ray Observatory (AMEGO) is a probe class mission concept that will provide essential contributions to multimessenger astrophysics in the late 2020s and beyond. AMEGO combines high sensitivity in the 200 keV to 10 GeV energy range with a wide field of view, good spectral resolution, and polarization sensitivity.
Petropoulou M, Yuan Y, Chen AY, Mastichiadis A. Inverse Compton Cascades in Pair-producing Gaps: Effects of Triplet Pair Production. [Internet]. 2019;883:66. WebsiteAbstract
Inverse Compton-pair cascades are initiated when gamma-rays are absorbed on an ambient soft photon field to produce relativistic pairs, which in turn up-scatter the same soft photons to produce more gamma-rays. If the Compton scatterings take place in the deep Klein-Nishina regime, then triplet pair production (e{γ }b\to {{ee}}+{e}-) becomes relevant and may even regulate the development of the cascade. We investigate the properties of pair-Compton cascades with triplet pair production in accelerating gaps, i.e., regions with an unscreened electric field. Using the method of transport equations for the particle evolution, we compute the growth rate of the pair cascade as a function of the accelerating electric field in the presence of blackbody and power-law ambient photon fields. Informed by the numerical results, we derive simple analytical expressions for the peak growth rate and the corresponding electric field. We show that for certain parameters, which can be realized in the vicinity of accreting supermassive black holes at the centers of active galactic nuclei, the pair cascade may well be regulated by inverse Compton scattering in the deep Klein-Nishina regime and triplet pair production. We present indicative examples of the escaping gamma-ray radiation from the gap, and discuss our results in application to the TeV observations of radio galaxy M87.
Ray P, Arzoumanian Z, Ballantyne D, Bozzo E, Brandt S, Brenneman L, Chakrabarty D, Christophersen M, DeRosa A, Feroci M, et al. STROBE-X: X-ray Timing and Spectroscopy on Dynamical Timescales from Microseconds to Years. In: Vol. 51. ; 2019. pp. 231. WebsiteAbstract
STROBE-X is a probe-class mission concept, selected for study by NASA, for X-ray spectral timing of compact objects across the mass scale. It combines huge collecting area, high throughput, broad energy coverage, and excellent spectral and temporal resolution in a single facility, enabling a broad portfolio of high-priority astrophysics.
Keivani A, Murase K, Petropoulou M, Fox DB, Cenko SB, Chaty S, Coleiro A, Delaunay JJ, Dimitrakoudis S, Evans PA, et al. VizieR Online Data Catalog: Swift follow-up obs. of the TXS 0506+056 blazar (Keivani+, 2018). [Internet]. 2019:J/ApJ/864/84. WebsiteAbstract
IceCube-170922A was an EHE neutrino event (GCN/AMON NOTICE 2016) identified and distributed by the IceCube Observatory via AMON and GCN within δt~43s of its detection at 20:54:30 UT on 2017 September 22 (GCN/AMON NOTICE IceCube-170922A 2017ApJ...843..109G). A refined localization was reported four hours later (Kopper & Blaufuss 2017GCN.21916....1K): RAJ2000=77.43-0.8+1.3°, DEJ2000=+5.72-0.4+0.7° (90% containment ellipse). IceCube-170922A triggered the Neil Gehrels Swift Observatory in automated fashion via AMON cyberinfrastructure, resulting in rapid-response mosaic-type follow-up observations, covering a roughly circular region of sky centered on the prompt localization in a 19-point tiling that began 3.25hr after the neutrino detection. This initial epoch of Swift observations spanned 22.5hr and accumulated ~800s exposure per pointing. The mosaic tiling yielded coverage of a region with radius ~0.8° centered on RAJ2000=05:09:08.784, DEJ2000=+05:45:13.32, amounting to a sky area of 2.1deg2. Source 2 from these observations (marked as X2 on Figure 1), located 4.6' from the center of the neutrino localization, was identified by us as the likely X-ray counterpart to QSO J0509+0541, also known as TXS 0506+056. This was the first report to connect TXS 0506+056 to IceCube-170922A (Keivani+ 2017GCN.21930....1K). Following the Fermi report that TXS 0506+056 was in a rare GeV-flaring state (Tanaka+ 2017ATel10791....1T), we commenced a Swift monitoring campaign on September 27 (Evans+ 2017ATel10792....1E). Swift monitored TXS 0506+056 for 36 epochs by November 30 with 53.7ks total exposure time (Table 1). The Swift-UVOT also participated in the rapid-response follow-up observations of the IceCube-170922A and the subsequent monitoring of the flaring blazar TXS 0506+056. (2 data files).
Yoshida K, Petropoulou M, Urry M, Coppi P, Bailyn C, Vasilopoulos G, Murase K, Oikonomou F. Flaring Rate Distribution of Gamma-Ray Blazars and Implications for High-Energy Neutrino Emission. In: Vol. 36. ; 2019. pp. 1038. Website
Petropoulou M, Sironi L, Spitkovsky A, Giannios D. Relativistic Magnetic Reconnection in Electron-Positron-Proton Plasmas: Implications for Jets of Active Galactic Nuclei. [Internet]. 2019;880:37. WebsiteAbstract
Magnetic reconnection is often invoked to explain the nonthermal radiation of relativistic outflows, including jets of active galactic nuclei (AGNs). Motivated by the largely unknown plasma composition of AGN jets, we study reconnection in the unexplored regime of electron-positron-proton (pair-proton) plasmas with large-scale two-dimensional particle-in-cell simulations. We cover a wide range of pair multiplicities (lepton-to-proton number ratio κ = 1-199) for different values of the all-species plasma magnetization (σ = 1, 3, and 10) and electron temperature ({{{\Theta }}}e\equiv {{kT}}e/{m}e{c}2=0.1{--}100). We focus on the dependence of the post-reconnection energy partition and lepton energy spectra on the hot pair plasma magnetization {σ }e,h (i.e., the ratio of magnetic to pair enthalpy densities). We find that the post-reconnection energy is shared roughly equally between magnetic fields, pairs, and protons for {σ }e,h ≳ 3. We empirically find that the mean lepton Lorentz factor in the post-reconnection region depends on σ, Θ e , and {σ }e,h as < {γ }e-1> ≈ \sqrt{σ }(1+4{{{\Theta }}}e)≤ft(1+{σ }e,h/30\right), for σ ≥ 1. The high-energy part of the post-reconnection lepton energy distributions can be described by a power law, whose slope is mainly controlled by {σ }e,h for κ ≳ 3-6, with harder power laws obtained for higher magnetizations. We finally show that reconnection in pair-proton plasmas with multiplicities κ ∼ 1-20, magnetizations σ ∼ 1-10, and temperatures Θ e ∼ 1-10 results in particle power-law slopes and average electron Lorentz factors that are consistent with those inferred in leptonic models of AGN jet emission.
Babic A, Hassan T, Paneque D, Balokovic M, Finke J, Petropoulou M. Unravelling the complex behavior of Mrk421 with simultaneous X-ray and VHE observations during an extreme flaring activity in April 2013. In: Vol. 36. ; 2019. pp. 624. Website
Venters T, Ajello M, Brandt TJ, Blumer H, Briggs M, Coppi P, D'Ammando F, Fields B, Finke J, Fryer C, et al. Energetic Particles of Cosmic Accelerators II: Active Galactic Nuclei and Gamma-ray Bursts. [Internet]. 2019;51:485. WebsiteAbstract
This white paper is the first of a two-part series highlighting the most well-known high-energy cosmic accelerators and contributions that MeV gamma- ray astronomy will bring to understanding their energetic particle phenomena. This white paper discusses active galactic nuclei and gamma-ray bursts.
Xue R, Liu R-Y, Petropoulou M, Oikonomou F, Wang Z-R, Wang K, Wang X-Y. A Two-zone Model for Blazar Emission: Implications for TXS 0506+056 and the Neutrino Event IceCube-170922A. [Internet]. 2019;886:23. WebsiteAbstract
A high-energy muon neutrino event, IceCube-170922A, was recently discovered in both spatial and temporal coincidence with a gamma-ray flare of the blazar TXS 0506+056. It has been shown with standard one-zone models that neutrinos can be produced in the blazar jet via hadronic interactions, but with a flux that is mostly limited by the X-ray data. In this work, we explore the neutrino production from TXS 0506+056 by invoking two physically distinct emission zones in the jet, with an inner blob inside of or close to the broad-line region (BLR) and an outer one well beyond the BLR. Using the Doppler-boosted radiation of the BLR as the target photon field, the inner zone accounts for the neutrino and gamma-ray emission via pγ interactions and inverse Compton scattering, respectively, while the outer zone produces the optical and X-ray emission via synchrotron and synchrotron self-Compton processes. The different conditions of the two zones allow us to suppress the X-ray emission from the electromagnetic cascade, and set a much higher upper limit on the muon neutrino flux (i.e., ∼10-11 erg cm-2 s-1) than in one-zone models. We compare our scenario in detail with one-zone models discussed in the literature, and argue that differentiating between such scenarios will become possible with next-generation neutrino telescopes, such as IceCube-Gen2.
2018
Murase K, Oikonomou F, Petropoulou M. Blazar Flares as an Origin of High-energy Cosmic Neutrinos?. [Internet]. 2018;865:124. WebsiteAbstract
We consider implications of high-energy neutrino emission from blazar flares, including the recent event IceCube-170922A and the 2014-2015 neutrino flare that could originate from TXS 0506+056. First, we discuss their contribution to the diffuse neutrino intensity taking into account various observational constraints. Blazars are likely to be subdominant in the diffuse neutrino intensity at sub-PeV energies, and we show that blazar flares like those of TXS 0506+056 could make ≲1%-10% of the total neutrino intensity. We also argue that the neutrino output of blazars can be dominated by the flares in the standard leptonic scenario for their γ-ray emission, and energetic flares may still be detected with a rate of ≲ 1 {yr}}-1. Second, we consider multi-messenger constraints on the source modeling. We show that luminous neutrino flares should be accompanied by luminous broadband cascade emission, emerging also in X-rays and γ-rays. This implies that not only γ-ray telescopes like Fermi but also X-ray sky monitors such as Swift and MAXI are critical to test the canonical picture based on the single-zone modeling. We also suggest a two-zone model that can naturally satisfy the X-ray constraints while explaining the flaring neutrinos via either photomeson or hadronuclear processes.
Keivani A, Murase K, Petropoulou M, Fox DB, Cenko SB, Chaty S, Coleiro A, Delaunay JJ, Dimitrakoudis S, Evans PA, et al. A Multimessenger Picture of the Flaring Blazar TXS 0506+056: Implications for High-energy Neutrino Emission and Cosmic-Ray Acceleration. [Internet]. 2018;864:84. WebsiteAbstract
Detection of the IceCube-170922A neutrino coincident with the flaring blazar TXS 0506+056, the first and only ∼3σ high-energy neutrino source association to date, offers a potential breakthrough in our understanding of high-energy cosmic particles and blazar physics. We present a comprehensive analysis of TXS 0506+056 during its flaring state, using newly collected Swift, NuSTAR, and X-shooter data with Fermi observations and numerical models to constrain the blazar’s particle acceleration processes and multimessenger (electromagnetic (EM) and high-energy neutrino) emissions. Accounting properly for EM cascades in the emission region, we find a physically consistent picture only within a hybrid leptonic scenario, with γ-rays produced by external inverse-Compton processes and high-energy neutrinos via a radiatively subdominant hadronic component. We derive robust constraints on the blazar’s neutrino and cosmic-ray emissions and demonstrate that, because of cascade effects, the 0.1-100 keV emissions of TXS 0506+056 serve as a better probe of its hadronic acceleration and high-energy neutrino production processes than its GeV-TeV emissions. If the IceCube neutrino association holds, physical conditions in the TXS 0506+056 jet must be close to optimal for high-energy neutrino production, and are not favorable for ultrahigh-energy cosmic-ray acceleration. Alternatively, the challenges we identify in generating a significant rate of IceCube neutrino detections from TXS 0506+056 may disfavor single-zone models, in which γ-rays and high-energy neutrinos are produced in a single emission region. In concert with continued operations of the high-energy neutrino observatories, we advocate regular X-ray monitoring of TXS 0506+056 and other blazars in order to test single-zone blazar emission models, clarify the nature and extent of their hadronic acceleration processes, and carry out the most sensitive possible search for additional multimessenger sources.
Petropoulou M, Mastichiadis A. Patterns of variability in supercritical hadronic systems. [Internet]. 2018;477:2917 - 2925. WebsiteAbstract
A unique and often overlooked property of a source loaded with relativistic protons is that it can become supercritical, i.e. it can undergo an abrupt transition from a radiatively inefficient to a radiatively efficient state once its proton energy density exceeds a certain threshold. In this paper, we investigate the temporal variability of hadronic systems in this hardly explored regime. We show that there exists a range of proton densities that prevent the system from reaching a steady state, but drive it instead in a quasi-periodic mode. The escaping radiation then exhibits limit cycles, even if all physical parameters are held constant in time. We extend our analysis to cases where the proton injection rate varies with time and explore the variability patterns of escaping radiation as the system moves in and out from the supercritical regime. We examine the relevance of our results to the variability of the prompt gamma-ray burst emission and show that, at least on a phenomenological level, some interesting analogies exist.
Meyer ET, Petropoulou M, Georganopoulos M, Chiaberge M, Breiding P, Sparks WB. Detection of an Optical/UV Jet/Counterjet and Multiple Spectral Components in M84. [Internet]. 2018;860:9. WebsiteAbstract
We report an optical/UV jet and counterjet in M84, previously unreported in archival Hubble Space Telescope imaging. With archival VLA, ALMA, and Chandra imaging, we examine the first well-sampled spectral energy distribution of the inner jet of M84, where we find that multiple co-spatial spectral components are required. In particular, the ALMA data reveal that the radio spectrum of all four knots in the jet turns over at approximately 100 GHz, which requires a second component for the bright optical/UV emission. Further, the optical/UV has a soft spectrum and is inconsistent with the relatively flat X-ray spectrum, which indicates a third component at higher energies. Using archival VLA imaging, we have measured the proper motion of the innermost knots at 0.9 ± 0.6 and 1.1 ± 0.4c, which when combined with the low jet-to-counterjet flux ratio yields an orientation angle for the system of {74}-18+9°. In the radio, we find high fractional polarization of the inner jet of up to 30% while in the optical no polarization is detected (<8%). We investigate different scenarios for explaining the particular multicomponent spectral energy distribution (SED) of the knots. Inverse Compton models are ruled out due to the extreme departure from equipartition and the unrealistically high total jet power required. The multicomponent SED can be naturally explained within a leptohadronic scenario, but at the cost of very high power in relativistic protons. A two-component synchrotron model remains a viable explanation, but more theoretical work is needed to explain the origin and properties of the electron populations.
Petropoulou M, Christie IM, Sironi L, Giannios D. Plasmoid statistics in relativistic magnetic reconnection. [Internet]. 2018;475:3797 - 3812. WebsiteAbstract
Plasmoids, overdense blobs of plasma containing magnetic fields and high-energy particles, are a self-consistent outcome of the reconnection process in the relativistic regime. Recent two-dimensional particle-in-cell (PIC) simulations have shown that plasmoids can undergo a variety of processes (e.g. mergers, bulk acceleration, growth, and advection) within the reconnection layer. We developed a Monte Carlo code, benchmarked with the recent PIC simulations, to examine the effects of these processes on the steady-state size and momentum distributions of the plasmoid chain. The differential plasmoid size distribution is shown to be a power law, ranging from a few plasma skin depths to ∼0.1 of the reconnection layer's length. The power-law slope is shown to be linearly dependent upon the ratio of the plasmoid acceleration and growth rates, which slightly decreases with increasing plasma magnetization. We perform a detailed comparison of our results with those of recent PIC simulations and briefly discuss the astrophysical implications of our findings through the representative case of flaring events from blazar jets.
Vasilopoulos G, Maitra C, Haberl F, Hatzidimitriou D, Petropoulou M. Identification of two new HMXBs in the LMC: an ∼2013 s pulsar and a probable SFXT. [Internet]. 2018;475:220 - 231. WebsiteAbstract
We report on the X-ray and optical properties of two high-mass X-ray binary systems located in the Large Magellanic Cloud (LMC). Based on the obtained optical spectra, we classify the massive companion as a supergiant star in both systems. Timing analysis of the X-ray events collected by XMM-Newton revealed the presence of coherent pulsations (spin period ∼2013 s) for XMMU J053108.3-690923 and fast flaring behaviour for XMMU J053320.8-684122. The X-ray spectra of both systems can be modelled sufficiently well by an absorbed power law, yielding hard spectra and high intrinsic absorption from the environment of the systems. Due to their combined X-ray and optical properties, we classify both systems as SgXRBs: the 19th confirmed X-ray pulsar and a probable supergiant fast X-ray transient in the LMC, the second such candidate outside our Galaxy.
Petropoulou M, Vasilopoulos G, Christie IM, Giannios D, Coe MJ. X-ray mapping of the stellar wind in the binary PSR J2032+4127/MT91 213. [Internet]. 2018;474:L22 - L26. WebsiteAbstract
PSR J2032+4127 is a young and rapidly rotating pulsar on a highly eccentric orbit around the high-mass Be star MT91 213. X-ray monitoring of the binary system over an ∼4000 d period with Swift has revealed an increase of the X-ray luminosity which we attribute to the synchrotron emission of the shocked pulsar wind. We use Swift X-ray observations to infer a clumpy stellar wind with r-2 density profile and constrain the Lorentz factor of the pulsar wind to 105 < γw < 106. We investigate the effects of an axisymmetric stellar wind with polar gradient on the X-ray emission. Comparison of the X-ray light curve hundreds of days before and after the periastron can be used to explore the polar structure of the wind.
Petropoulou M, Dimitrakoudis S, Padovani P, Resconi E, Giommi P, Mastichiadis A. The many faces of blazar emission in the context of hadronic models. In: ; 2018. pp. 3061 - 3067. WebsiteAbstract
We present two ways of modeling the spectral energy distribution of blazars in the hadronic context and discuss the predictions of each "hadronic variant", the spectral shape, the multi-wavelength variability, the cosmic-ray flux, and the high-energy neutrino emission. Focusing on the latter, we then present an application of the hadronic model to individual BL Lacs that were recently suggested to be the counterparts of some of the IceCube neutrinos, and conclude by discussing the contribution of the whole BL Lac class to the observed neutrino background.
Meyer E, Georganopoulos M, Petropoulou M, Breiding P. Multiple Spectral Components in Large-Scale Jets. In: Vol. 231. ; 2018. pp. 323.02. WebsiteAbstract
One of the most striking discoveries of the Chandra X-ray Observatory is the population of bright X-ray emitting jets hosted by powerful quasars. Most of these jets show hard X-ray spectra which requires a separate spectral component compared with the radio-optical synchrotron emission, which usually peaks at or before the infrared. Though the origin of this high-energy spectral component has been a matter of debate for nearly two decades, it is still not understood, with major implications for our understanding of particle acceleration in jets, as well as the total energy carried by them. Until recently the prevailing interpretation for the second component has been inverse-Compont upscattering of the CMB by a still highly relativistic jet at kpc scales. I will briefly describe the recent work calling the IC/CMB model into serious question (including X-ray variability, UV polarization, gamma-ray upper limits, and proper motions), and present new results, based on new ALMA, HST, and Chandra observations, which suggest that more than two distinct spectral components may be present in some large-scale jets, and that these multiple components appear to arise in jets across the full range in jet power, and not just in the most powerful sources. These results are very difficult to reconcile with simple models of jet emission, and I will discuss these failures and some possible directions for the future, including hadronic models.
Boula S, Petropoulou M, Mastichiadis A. On the Connection of Radio and γ-Ray Emission in Blazars. [Internet]. 2018;7:3. WebsiteAbstract
Blazars are a sub-category of radio-loud active galactic nuclei with relativistic jets pointing towards to the observer. They are well-known for their non-thermal variable emission, which practically extends over the whole electromagnetic spectrum. Despite the plethora of multi-wavelength observations, the issue about the origin of the γ -ray and radio emission in blazar jets remains unsettled. Here, we construct a parametric leptonic model for studying the connection between the γ -ray and radio emission in both steady-state and flaring states of blazars. Assuming that relativistic electrons are injected continuously at a fixed distance from the black hole, we numerically study the evolution of their population as it propagates to larger distances while losing energy due to expansion and radiative cooling. In this framework, γ -ray photons are naturally produced at small distances (e.g., 10 - 3 pc) when the electrons are still very energetic, whereas the radio emission is produced at larger distances (e.g., 1 pc), after the electrons have cooled and the emitting region has become optically thin to synchrotron self-absorption due to expansion. We present preliminary results of our numerical investigation for the steady-state jet emission and the predicted time lags between γ -rays and radio during flares.
Dong L, Petropoulou M, Giannios D. Extreme scattering events from axisymmetric plasma lenses. [Internet]. 2018;481:2685 - 2693. WebsiteAbstract
Frequency-dependent brightness fluctuations of radio sources, the so-called extreme scattering events (ESEs), have been observed over the last three decades. They are caused by Galactic plasma structures whose geometry and origin are still poorly understood. In this paper, we construct axisymmentric two-dimensional (2D) column density profiles for the plasma lens and explore the resulting ESEs for both point-like and extended sources. A quantity that becomes relevant is the impact parameter b, namely the distance of the observer's path from the lens' symmetry axis. We demonstrate its effects on the shape of ESE light curves and use it for a phenomenological classification of ESEs into four main types. Three of them are unique outcomes of the 2D model and do not show a characteristic U-shaped dip in the light curve, which has been traditionally used as an identification means of ESEs. We apply our model to five well-studied ESEs and show that elongated plasma tubes or quasi-spherical clouds are favoured over plasma sheets for four of them, while the remaining one is compatible with both lens geometries.
Petropoulou M, Sironi L. The steady growth of the high-energy spectral cut-off in relativistic magnetic reconnection. [Internet]. 2018;481:5687 - 5701. WebsiteAbstract
Magnetic reconnection is invoked as an efficient particle accelerator in a variety of astrophysical sources of non-thermal high-energy radiation. With large-scale two-dimensional particle-in-cell simulations of relativistic reconnection (i.e. with magnetization σ ≫ 1) in pair plasmas, we study the long-term evolution of the power-law slope and high-energy cut-off of the spectrum of accelerated particles. We find that the high-energy spectral cut-off does not saturate at γcut ∼ 4σ, as claimed by earlier studies, but it steadily grows with time as long as the reconnection process stays active. At late times, the cut-off scales approximately as γ _cut∝ √{t}, regardless of the flow magnetization and initial temperature. We show that the particles dominating the high-energy spectral cut-off reside in plasmoids, and in particular in a strongly magnetized ring around the plasmoid core. The growth of their energy is driven by the increase in the local field strength, coupled with the conservation of the first adiabatic invariant. We also find that the power-law slope of the spectrum (p = -d log N/d log γ) evolves with time. For σ ≳ 10, the spectrum is hard at early times (p ≲ 2), but it tends to asymptote to p ∼ 2; the steepening of the power-law slope allows the spectral cut-off to extend to higher and higher energies, without violating the fixed energy budget of the system. Our results demonstrate that relativistic reconnection is a viable candidate for accelerating the high-energy particles emitting in relativistic astrophysical sources.
2017
Christie I, Petropoulou M, Sironi L, Giannios D. Blazar Variability from Plasmoids in Relativistic Reconnection. In: ; 2017. pp. 40. Website
Petropoulou M. Anatomy of a gamma-ray burst. [Internet]. 2017;1:567 - 568. WebsiteAbstract
Linearly polarized optical emission from a gamma-ray burst reveals the presence of a large-scale distorted magnetic field in the heart of this powerful cosmic explosion.
Petropoulou M, Coenders S, Vasilopoulos G, Kamble A, Sironi L. Point-source and diffuse high-energy neutrino emission from Type IIn supernovae. [Internet]. 2017;470:1881 - 1893. WebsiteAbstract
Type IIn supernovae (SNe), a rare subclass of core collapse SNe, explode in dense circumstellar media that have been modified by the SNe progenitors at their last evolutionary stages. The interaction of the freely expanding SN ejecta with the circumstellar medium gives rise to a shock wave propagating in the dense SN environment, which may accelerate protons to multi-PeV energies. Inelastic proton-proton collisions between the shock-accelerated protons and those of the circumstellar medium lead to multimessenger signatures. Here, we evaluate the possible neutrino signal of Type IIn SNe and compare with IceCube observations. We employ a Monte Carlo method for the calculation of the diffuse neutrino emission from the SN IIn class to account for the spread in their properties. The cumulative neutrino emission is found to be ∼10 per cent of the observed IceCube neutrino flux above 60 TeV. Type IIn SNe would be the dominant component of the diffuse astrophysical flux, only if 4 per cent of all core collapse SNe were of this type and 20-30 per cent of the shock energy was channeled to accelerated protons. Lower values of the acceleration efficiency are accessible by the observation of a single Type IIn SN as a neutrino point source with IceCube using up-going muon neutrinos. Such an identification is possible in the first year following the SN shock breakout for sources within 20 Mpc.
Nalewajko K, Hayashida M, Madejski G, Petropoulou M. The challenge of rapid gamma-ray variability of blazar 3C 279. In: Vol. 16. ; 2017. pp. 106.23. WebsiteAbstract
Detections of gamma-ray variability of active galaxies on time scales of a few minutes revealed the most extreme regimes of dissipation and particle acceleration in relativistic plasmas. Observations of blazar 3C 279 by the Fermi Large Area Telescope during a successful Target-of-Opportunity pointing campaign in June 2015 detected very clearly and for the first time variability in the GeV band on time scales 5 minutes and possibly shorter. This result presents a unique challenge for the theory of relativistic jets, since 3C 279 is also a quasar with dense radiative environment that can readily absorb gamma rays produced at sub-pc distance scales. The parameters required to explain such variability are extreme, regardless of the assumption of the radiation mechanism (inverse Compton, synchrotron, lepto-hadronic). Very high bulk Lorentz factors, Gamma ~ 100, and kinetic beaming effect of relativistic magnetic reconnection are proposed as ingredients of a complete solution to this problem that remains elusive.
Christie IM, Petropoulou M, Mimica P, Giannios D. Radio emission from Sgr A*: pulsar transits through the accretion disc. [Internet]. 2017;468:L26 - L30. WebsiteAbstract
Radiatively inefficient accretion flow models have been shown to accurately account for the spectrum and luminosity observed from Sgr A* in the X-ray regime down to mm wavelengths. However, observations at a few GHz cannot be explained by thermal electrons alone but require the presence of an additional non-thermal particle population. Here, we propose a model for the origin of such a population in the accretion flow via means of a pulsar orbiting the supermassive black hole in our Galaxy. Interactions between the relativistic pulsar wind with the disc lead to the formation of a bow shock in the wind. During the pulsar's transit through the accretion disc, relativistic pairs, accelerated at the shock front, are injected into the disc. The radio-emitting particles are long lived and remain within the disc long after the pulsar's transit. Periodic pulsar transits through the disc result in regular injection episodes of non-thermal particles. We show that for a pulsar with spin-down luminosity Lsd ∼ 3 × 1035 erg s-1 and a wind Lorentz factor of γw ∼ 104 a quasi-steady synchrotron emission is established with luminosities in the 1-10 GHz range comparable to the observed one.
Petropoulou M, Nalewajko K, Hayashida M, Mastichiadis A. A hadronic minute-scale GeV flare from quasar 3C 279?. [Internet]. 2017;467:L16 - L20. WebsiteAbstract
The flat spectrum radio quasar 3C 279 is a known γ-ray variable source that has recently exhibited minute-scale variability at energies >100 MeV. One-zone leptonic models for blazar emission are severely constrained by the short time-scale variability that implies a very compact emission region at a distance of hundreds of Schwarzschild radii from the central black hole. Here, we investigate a hadronic scenario where GeV γ-rays are produced via proton synchrotron radiation. We also take into account the effects of the hadronically initiated electromagnetic cascades (EMC). For a γ-ray emitting region in rough equipartition between particles and kG magnetic fields, located within the broad-line region (BLR), the development of EMC redistributes the γ-ray luminosity to softer energy bands and eventually leads to broad-band spectra that differ from the observed ones. Suppression of EMC and energy equipartition are still possible, if the γ-ray emitting region is located beyond the BLR, is fast moving with Doppler factor (>70) and contains strong magnetic fields (>100 G). Yet, these conditions cannot be easily met in parsec-scale jets, thus disfavouring a proton synchrotron origin of the Fermi-LAT flare.
Petropoulou M, Vasilopoulos G, Giannios D. The TeV emission of Ap Librae: a hadronic interpretation and prospects for CTA. [Internet]. 2017;464:2213 - 2222. WebsiteAbstract
Ap Librae is one out of a handful of low-frequency peaked blazars to be detected at TeV γ-rays and the only one with an identified X-ray jet. Combined observations of Fermi-LAT at high energies (HE) and of H.E.S.S. at very high energies (VHE) revealed a striking spectral property of Ap Librae; the presence of a broad high-energy component that extends more than nine orders of magnitude in energy and is, therefore, hard to be explained by the usual single-zone synchrotron self-Compton model. We show that the superposition of different emission components related to photohadronic interactions can explain the γ-ray emission of Ap Librae without invoking external radiation fields. We present two indicative model fits to the spectral energy distribution of Ap Librae where the VHE emission is assumed to originate from a compact, sub-pc scale region of the jet. A robust prediction of our model is VHE flux variability on time-scales similar to those observed at X-rays and HE γ-rays, which can be further used to distinguish between a sub-pc or kpc scale origin of the TeV emission. We thus calculate the expected variability signatures at X-rays, HE and VHE γ-rays and show that quasi-simultaneous flares are expected, with larger amplitude flares appearing at γ-rays. We assess the detectability of VHE variability from Ap Librae with CTA, next generation of IACT. We show that ∼h time-scale variability at Eγ > 0.1 TeV could be detectable at high significance with shorter exposure times than current Cherenkov telescopes.
Petropoulou M, Duran RB, Giannios D. Collapsar γ-ray bursts: how the luminosity function dictates the duration distribution. [Internet]. 2017;472:2722 - 2727. WebsiteAbstract
Jets in long-duration γ-ray bursts (GRBs) have to drill through the collapsing star in order to break out of it and produce the γ-ray signal while the central engine is still active. If the breakout time is shorter for more powerful engines, then the jet-collapsar interaction acts as a filter of less luminous jets. We show that the observed broken power-law GRB luminosity function is a natural outcome of this process. For a theoretically motivated breakout time that scales with jet luminosity as L-χ with χ ∼ 1/3-1/2, we show that the shape of the γ-ray duration distribution can be uniquely determined by the GRB luminosity function and matches the observed one. This analysis has also interesting implications about the supernova-central engine connection. We show that not only successful jets can deposit sufficient energy in the stellar envelope to power the GRB-associated supernovae, but also failed jets may operate in all Type Ib/c supernovae.
2016
Christie IM, Petropoulou M, Mimica P, Giannios D. Modelling accretion disc and stellar wind interactions: the case of Sgr A*. [Internet]. 2016;459:2420 - 2431. WebsiteAbstract
Sgr A* is an ideal target to study low-luminosity accreting systems. It has been recently proposed that properties of the accretion flow around Sgr A* can be probed through its interactions with the stellar wind of nearby massive stars belonging to the S-cluster. When a star intercepts the accretion disc, the ram and thermal pressures of the disc terminate the stellar wind leading to the formation of a bow shock structure. Here, a semi-analytical model is constructed which describes the geometry of the termination shock formed in the wind. With the employment of numerical hydrodynamic simulations, this model is both verified and extended to a region prone to Kelvin-Helmholtz instabilities. Because the characteristic wind and stellar velocities are in ∼108 cm s-1 range, the shocked wind may produce detectable X-rays via thermal bremsstrahlung emission. The application of this model to the pericentre passage of S2, the brightest member of the S-cluster, shows that the shocked wind produces roughly a month long X-ray flare with a peak luminosity of L ≈ 4 × 1033 erg s-1 for a stellar mass-loss rate, disc number density, and thermal pressure strength of dot{M}_w= 10^{-7} M_{☉} yr^{-1}, nd = 105 cm-3, and α = 0.1, respectively. This peak luminosity is comparable to the quiescent X-ray emission detected from Sgr A* and is within the detection capabilities of current X-ray observatories. Its detection could constrain the density and thickness of the disc at a distance of ∼3000 gravitational radii from the supermassive black hole.
Petropoulou M, Dermer CD. Properties of Blazar Jets Defined by an Economy of Power. [Internet]. 2016;825:L11. WebsiteAbstract
The absolute power of a relativistic black hole jet includes the power in the magnetic field, the leptons, the hadrons, and the radiated photons. A power analysis of a relativistic radio/γ-ray blazar jet leads to bifurcated leptonic synchrotron-Compton (LSC) and leptohadronic synchrotron (LHS) solutions that minimize the total jet power. Higher Doppler factors with increasing peak synchrotron frequency are implied in the LSC model. Strong magnetic fields {B}\prime ≳ 100 {{G}} are found for the LHS model with variability times ≲ {10}3 {{s}}, in accord with highly magnetized, reconnection-driven jet models. Proton synchrotron models of ≳ 100 {GeV} blazar radiation can have sub-Eddington absolute jet powers, but models of dominant GeV radiation in flat spectrum radio quasars require excessive power.
Petropoulou M, Kamble A, Sironi L. Radio synchrotron emission from secondary electrons in interaction-powered supernovae. [Internet]. 2016;460:44 - 66. WebsiteAbstract
Several supernovae (SNe) with an unusually dense circumstellar medium (CSM) have been recently observed at radio frequencies. Their radio emission is powered by relativistic electrons that can be either accelerated at the SN shock (primaries) or injected as a by-product (secondaries) of inelastic proton-proton collisions. We investigate the radio signatures from secondary electrons, by detailing a semi-analytical model to calculate the temporal evolution of the distributions of protons, primary and secondary electrons. With our formalism, we track the cooling history of all the particles that have been injected into the emission region up to a given time, and calculate the resulting radio spectra and light curves. For an SN shock propagating through the progenitor wind, we find that secondary electrons control the early radio signatures, but their contribution decays faster than that of primary electrons. This results in a flattening of the light curve at a given radio frequency that depends only upon the radial profiles of the CSM density and of the shock velocity, υ0. The relevant transition time at the peak frequency is {∼ } {190} d K_ep,-3^{-1} A_{w, 16}{/β _{0, -1.5}^2}, where Aw is the wind mass-loading parameter, β0 = υ0/c and Kep are the electron-to-proton ratio of accelerated particles. We explicitly show that late peak times at 5 GHz (I.e. tpk ≳ 300-1000 d) suggest a shock wave propagating in a dense wind (Aw ≳ 1016-1017 gr cm-1), where secondary electrons are likely to power the observed peak emission.
Petropoulou M, Coenders S, Dimitrakoudis S. Time-dependent neutrino emission from Mrk 421 during flares and predictions for IceCube. [Internet]. 2016;80:115 - 130. WebsiteAbstract
Blazars, a subclass of active galactic nuclei, are prime candidate sources for the high energy neutrinos recently detected by IceCube. Being one of the brightest sources in the extragalactic X-ray and γ-ray sky as well as one of the nearest blazars to Earth, Mrk 421 is an excellent source for testing the scenario of the blazar-neutrino connection, especially during flares where time-dependent neutrino searches may have a higher detection probability. Here, we model the spectral energy distribution of Mrk 421 during a 13-day flare in 2010 with unprecedented multi-wavelength coverage, and calculate the respective neutrino flux. We find a correlation between the >1 PeV neutrino and photon fluxes, in all energy bands. Using typical IceCube through-going muon event samples with good angular resolution and high statistics, wederive the mean event rate above 100 TeV (∼0.57 evt/yr) and show that it is comparable to that expected from a four-month quiescent period in 2009. Due to the short duration of the flare, an accumulation of similar flares over several years would be necessary to produce a meaningful signal for IceCube. To better assess this, we apply the correlation between the neutrino and γ-ray fluxes to the 6.9 yr Fermi-LAT light curve of Mrk 421. We find that the mean event count above 1 PeV for the full IceCube detector livetime is 3.59 ± 0.60 (2.73 ± 0.38) νμ +νbarμ with (without) major flares included in our analysis. This estimate exceeds, within the uncertainties, the 95% (90%) threshold value for the detection of one or more muon (anti-)neutrinos. Meanwhile, the most conservative scenario, where no correlation of γ-rays and neutrinos is assumed, predicts 1.60 ± 0.16νμ +νbarμ events. We conclude that a non-detection of high-energy neutrinos by IceCube would probe the neutrino/γ-ray flux correlation during major flares or/and the hadronic contribution to the blazar emission.
Kantzas D, Petropoulou M, Mastichiadis A. Early-time signatures of γ-ray emission from supernovae in dense circumstellar media. In: ; 2016. pp. 139. WebsiteAbstract
We present our results on the γ-ray emission from interaction-powered supernovae (SNe), a recently discovered SN type that is suggested to be surrounded by a circumstellar medium (CSM) with densities 10^7-10^12~ cm^-3. Such high densities favor inelastic collisions between relativistic protons accelerated in the SN blast wave and CSM protons and the production of γ-ray photons through neutral pion decays. Using a numerical code that includes synchrotron radiation, adiabatic losses due to the expansion of the source, photon-photon interactions, proton-proton collisions and proton-photon interactions, we calculate the multi-wavelength non-thermal photon emission soon after the shock breakout and follow its temporal evolution until 100-1000 days. Focusing on the γ-ray emission at >100 MeV, we show that this could be detectable by the Fermi-LAT telescope for nearby (<10 Mpc) SNe with dense CSM (>10^11 cm^(-3)).
Vasilopoulos G, Petropoulou M. The X-ray dust-scattered rings of the black hole low-mass binary V404 Cyg. [Internet]. 2016;455:4426 - 4441. WebsiteAbstract
We report on the first detection of X-ray dust-scattered rings from the Galactic low-mass X-ray binary V404 Cyg. The observation of the system with Swift/XRT on 2015 June 30 revealed the presence of five concentric ring-like structures centred at the position of V404 Cyg. Follow-up Swift/XRT observations allowed a time-dependent study of the X-ray rings. Assuming that these are the result of small-angle, single X-ray scattering by dust grains along the line of sight, we find that their angular size scales as θ ∝ √{t} in agreement with theoretical predictions. The dust grains are concentrated in five dust layers located at about 2.12, 2.05, 1.63, 1.50 and 1.18 kpc from the observer. These coincide roughly with locations of enhanced extinction as determined by infrared photometry. Assuming that the grain size distribution is described by a generalized Mathis-Rumpl-Nordsieck model, we find that the power-law index of the most distant cloud is q ∼ 4.4, while q ∼ 3.5-3.7 in all other clouds. We constrain at a 3σ level the maximum grain size of the intermediate dust layers in the range 0.16-0.20 μm and set a lower limit of ∼ 0.2 μm in the other clouds. Hints of an exponential cutoff at the angular intensity profile of the outermost X-ray ring suggest that the smallest grains have sizes 0.01 ≤ αmin ≲ 0.03 μm. Based on the relative ratios of dust column densities we find the highest dust concentration at ∼1.6 kpc. Our results indicate a gradient in the dust properties within 1 kpc from V404 Cyg.
Petropoulou M, Dimitrakoudis S, Padovani P, Resconi E, Giommi P, Mastichiadis A. The many faces of blazar emission in the context of hadronic models. [Internet]. 2016:arXiv:1601.06010. WebsiteAbstract
We present two ways of modeling the spectral energy distribution of blazars in the hadronic context and discuss the predictions of each "hadronic variant" on the spectral shape, the multi-wavelength variability, the cosmic-ray flux, and the high-energy neutrino emission. Focusing on the latter, we then present an application of the hadronic model to individual BL Lacs that were recently suggested to be the counterparts of some of the IceCube neutrinos, and conclude by discussing the contribution of the whole BL Lac class to the observed neutrino background.
Petropoulou M, Giannios D, Sironi L. Blazar flares powered by plasmoids in relativistic reconnection. [Internet]. 2016;462:3325 - 3343. WebsiteAbstract
Powerful flares from blazars with short (∼min) variability time-scales are challenging for current models of blazar emission. Here, we present a physically motivated ab initio model for blazar flares based on the results of recent particle-in-cell (PIC) simulations of relativistic magnetic reconnection. PIC simulations demonstrate that quasi-spherical plasmoids filled with high-energy particles and magnetic fields are a self-consistent by-product of the reconnection process. By coupling our PIC-based results (I.e. plasmoid growth, acceleration profile, particle and magnetic content) with a kinetic equation for the evolution of the electron distribution function we demonstrate that relativistic reconnection in blazar jets can produce powerful flares whose temporal and spectral properties are consistent with the observations. In particular, our model predicts correlated synchrotron and synchrotron self-Compton flares of duration of several hours-days powered by the largest and slowest moving plasmoids that form in the reconnection layer. Smaller and faster plasmoids produce flares of sub-hour duration with higher peak luminosities than those powered by the largest plasmoids. Yet, the observed fluence in both types of flares is similar. Multiple flares with a range of flux-doubling time-scales (minutes to several hours) observed over a longer period of flaring activity (days or longer) may be used as a probe of the reconnection layer's orientation and the jet's magnetization. Our model shows that blazar flares are naturally expected as a result of magnetic reconnection in a magnetically dominated jet.
Padovani P, Resconi E, Giommi P, Petropoulou M, et al. Blazars as Neutrinos Emitters. In: ; 2016. pp. 55. Website
Sironi L, Giannios D, Petropoulou M. Plasmoids in relativistic reconnection, from birth to adulthood: first they grow, then they go. [Internet]. 2016;462:48 - 74. WebsiteAbstract
Blobs, or quasi-spherical emission regions containing relativistic particles and magnetic fields, are often assumed ad hoc in emission models of relativistic astrophysical jets, yet their physical origin is still not well understood. Here, we employ a suite of large-scale 2D particle-in-cell simulations in electron-positron plasmas to demonstrate that relativistic magnetic reconnection can naturally account for the formation of quasi-spherical plasmoids filled with high-energy particles and magnetic fields. Our simulations extend to unprecedentedly long temporal and spatial scales, so we can capture the asymptotic physics independently of the initial setup. We characterize the properties of the plasmoids, continuously generated as a self-consistent by-product of the reconnection process: they are in rough energy equipartition between particles and magnetic fields; the upper energy cutoff of the plasmoid particle spectrum is proportional to the plasmoid width w, corresponding to a Larmor radius ∼0.2 w; the plasmoids grow in size at ∼0.1 of the speed of light, with most of the growth happening while they are still non-relativistic (first they grow'); their growth is suppressed once they get accelerated to relativistic speeds by the field line tension, up to the Alfvén speed (then they go'). The largest plasmoids reach a width wmax ∼ 0.2 L independently of the system length L, they have nearly isotropic particle distributions and contain the highest energy particles, whose Larmor radius is ∼0.03 L. The latter can be regarded as the Hillas criterion for relativistic reconnection. We briefly discuss the implications of our results for the high-energy emission from relativistic jets and pulsar winds.
2015
Petropoulou M, Dimitrakoudis S, Padovani P, Resconi E, Giommi P, Mastichiadis A. On the neutrino emission from BL Lacs. In: Vol. 34. ; 2015. pp. 1125. Website
Gazeas K, Vasilopoulos G, Petropoulou M, Sapountzis K. Optical follow-up of V404 Cyg during the current enhanced activity. [Internet]. 2015;7650:1. WebsiteAbstract
V404 Cyg is a known black hole Low mass X-ray binary with a late G-type companion, having a ~6.5 d orbital period. On June 15 18:32 UT Swift Burst Alert Telescope (BAT) was triggered due to the high X-ray activity of the system (Barthelmy et al.
Sironi L, Petropoulou M, Giannios D. Relativistic jets shine through shocks or magnetic reconnection?. [Internet]. 2015;450:183 - 191. WebsiteAbstract
Observations of gamma-ray-bursts and jets from active galactic nuclei reveal that the jet flow is characterized by a high radiative efficiency and that the dissipative mechanism must be a powerful accelerator of non-thermal particles. Shocks and magnetic reconnection have long been considered as possible candidates for powering the jet emission. Recent progress via fully-kinetic particle-in-cell simulations allows us to revisit this issue on firm physical grounds. We show that shock models are unlikely to account for the jet emission. In fact, when shocks are efficient at dissipating energy, they typically do not accelerate particles far beyond the thermal energy, and vice versa. In contrast, we show that magnetic reconnection can deposit more than 50 per cent of the dissipated energy into non-thermal leptons as long as the energy density of the magnetic field in the bulk flow is larger than the rest-mass energy density. The emitting region, i.e. the reconnection downstream, is characterized by a rough energy equipartition between magnetic fields and radiating particles, which naturally accounts for a commonly observed property of blazar jets.
Hovatta T, Petropoulou M, Richards JL, Giannios D, Wiik K, Baloković M, Lähteenmäki A, Lott B, Max-Moerbeck W, Ramakrishnan V, et al. A combined radio and GeV γ-ray view of the 2012 and 2013 flares of Mrk 421. [Internet]. 2015;448:3121 - 3131. WebsiteAbstract
In 2012 Markarian 421 underwent the largest flare ever observed in this blazar at radio frequencies. In the present study, we start exploring this unique event and compare it to a less extreme event in 2013. We use 15 GHz radio data obtained with the Owens Valley Radio Observatory 40-m telescope, 95 GHz millimetre data from the Combined Array for Research in Millimeter-Wave Astronomy, and GeV γ-ray data from the Fermi Gamma-ray Space Telescope. The radio light curves during the flaring periods in 2012 and 2013 have very different appearances, in both shape and peak flux density. Assuming that the radio and γ-ray flares are physically connected, we attempt to model the most prominent sub-flares of the 2012 and 2013 activity periods by using the simplest possible theoretical framework. We first fit a one-zone synchrotron self-Compton (SSC) model to the less extreme 2013 flare and estimate parameters describing the emission region. We then model the major γ-ray and radio flares of 2012 using the same framework. The 2012 γ-ray flare shows two distinct spikes of similar amplitude, so we examine scenarios associating the radio flare with each spike in turn. In the first scenario, we cannot explain the sharp radio flare with a simple SSC model, but we can accommodate this by adding plausible time variations to the Doppler beaming factor. In the second scenario, a varying Doppler factor is not needed, but the SSC model parameters require fine-tuning. Both alternatives indicate that the sharp radio flare, if physically connected to the preceding γ-ray flares, can be reproduced only for a very specific choice of parameters.
Petropoulou M, Dimitrakoudis S, Padovani P, Mastichiadis A, Resconi E. Photohadronic origin of γ -ray BL Lac emission: implications for IceCube neutrinos. [Internet]. 2015;448:2412 - 2429. WebsiteAbstract
The recent IceCube discovery of 0.1-1 PeV neutrinos of astrophysical origin opens up a new era for high-energy astrophysics. Although there are various astrophysical candidate sources, a firm association of the detected neutrinos with one (or more) of them is still lacking. A recent analysis of plausible astrophysical counterparts within the error circles of IceCube events showed that likely counterparts for nine of the IceCube neutrinos include mostly BL Lacs, among which Mrk 421. Motivated by this result and a previous independent analysis on the neutrino emission from Mrk 421, we test the BL Lac-neutrino connection in the context of a specific theoretical model for BL Lac emission. We model the spectral energy distribution (SED) of the BL Lacs selected as counterparts of the IceCube neutrinos using a one-zone leptohadronic model and mostly nearly simultaneous data. The neutrino flux for each BL Lac is self-consistently calculated, using photon and proton distributions specifically derived for every individual source. We find that the SEDs of the sample, although different in shape and flux, are all well fitted by the model using reasonable parameter values. Moreover, the model-predicted neutrino flux and energy for these sources are of the same order of magnitude as those of the IceCube neutrinos. In two cases, namely Mrk 421 and 1H 1914-194, we find a suggestively good agreement between the model prediction and the detected neutrino flux. Our predictions for all the BL Lacs of the sample are in the range to be confirmed or disputed by IceCube in the next few years of data sampling.
Petropoulou M, Mastichiadis A. Bethe-Heitler emission in BL Lacs: filling the gap between X-rays and γ-rays. [Internet]. 2015;447:36 - 48. WebsiteAbstract
We present the spectral signatures of the Bethe-Heitler pair production (pe) process on the spectral energy distribution (SED) of blazars, in scenarios where the hard γ-ray emission is of photohadronic origin. If relativistic protons interact with the synchrotron blazar photons producing γ rays through photopion processes, we show that, besides the ∼2-20 PeV neutrino emission, the typical blazar SED should have an emission feature due to the synchrotron emission of pe secondaries that bridges the gap between the low- and high-energy humps of the SED, namely in the energy range 40 keV-40 MeV. We first present analytical expressions for the photopion and pe loss rates in terms of observable quantities of blazar emission. For the pe loss rate in particular, we derive a new approximate analytical expression for the case of a power-law photon distribution, which has an excellent accuracy with the numerically calculated exact one, especially at energies much above the threshold for pair production. We show that for typical blazar parameters, the photopair synchrotron emission emerges in the hard X-ray/soft γ-ray energy range with a characteristic spectral shape and non-negligible flux, which may even be comparable to the hard γ-ray flux produced through photopion processes. We argue that the expected pe bumps' are a natural consequence of leptohadronic models, and as such, they may indicate that blazars with a three-hump SED are possible emitters of high-energy neutrinos.
Petropoulou M, Dimitrakoudis S. Constraints of flat spectrum radio quasars in the hadronic model: the case of 3C 273. [Internet]. 2015;452:1303 - 1315. WebsiteAbstract
We present a method of constraining the properties of the γ-ray emitting region in flat spectrum radio quasars in the one-zone proton synchrotron model, where the γ-rays are produced by synchrotron radiation of relativistic protons. We show that for low enough values of the Doppler factor δ, the emission from the electromagnetic (EM) cascade which is initiated by the internal absorption of high-energy photons from photohadronic interactions may exceed the observed ∼GeV flux. We use that effect to derive an absolute lower limit of δ; first, an analytical one, in the asymptotic limit where the external radiation from the broad-line region (BLR) is negligible, and then a numerical one in the more general case that includes BLR radiation. As its energy density in the emission region depends on δ and the region's distance from the galactic centre, we use the EM cascade to determine a minimum distance for each value of δ. We complement the EM cascade constraint with one derived from variability arguments and apply our method to the FSRQ 3C 273. We find that δ ≳ 18-20 for B ≲ 30 G and ∼day time-scale variability; the emission region is located outside the BLR, namely at r ≳ 10RBLR ∼ 3 pc; the model requires at pc-scale distances stronger magnetic fields than those inferred from core shift observations; while the jet power exceeds by at least one order of magnitude the accretion power. In short, our results disfavour the proton synchrotron model for the FSRQ 3C 273.
Padovani P, Petropoulou M, Giommi P, Resconi E. A simplified view of blazars: the neutrino background. [Internet]. 2015;452:1877 - 1887. WebsiteAbstract
Blazars have been suggested as possible neutrino sources long before the recent IceCube discovery of high-energy neutrinos. We re-examine this possibility within a new framework built upon the blazar simplified view and a self-consistent modelling of neutrino emission from individual sources. The former is a recently proposed paradigm that explains the diverse statistical properties of blazars adopting minimal assumptions on blazars' physical and geometrical properties. This view, tested through detailed Monte Carlo simulations, reproduces the main features of radio, X-ray, and γ-ray blazar surveys and also the extragalactic γ-ray background at energies ≳ 10 GeV. Here, we add a hadronic component for neutrino production and estimate the neutrino emission from BL Lacertae objects as a class, calibrated' by fitting the spectral energy distributions of a preselected sample of such objects and their (putative) neutrino spectra. Unlike all previous papers on this topic, the neutrino background is then derived by summing up at a given energy the fluxes of each BL Lac in the simulation, all characterized by their own redshift, synchrotron peak energy, γ-ray flux, etc. Our main result is that BL Lacs as a class can explain the neutrino background seen by IceCube above ∼0.5 PeV while they only contribute ∼10 per cent at lower energies, leaving room to some other population(s)/physical mechanism. However, one cannot also exclude the possibility that individual BL Lacs still make a contribution at the ≈20 per cent level to the IceCube low-energy events. Our scenario makes specific predictions, which are testable in the next few years.
Petropoulou M, Piran T, Mastichiadis A. Spectral signatures of compact sources in the inverse Compton catastrophe limit. [Internet]. 2015;452:3226 - 3245. WebsiteAbstract
The inverse Compton catastrophe is defined as a dramatic rise in the luminosity of inverse Compton scattered photons. It is described by a non-linear loop of radiative processes that sets in for high values of the electron compactness and is responsible for the efficient transfer of energy from electrons to photons, predominantly through inverse Compton scatterings. We search for the conditions that drive a magnetized non-thermal source to the inverse Compton catastrophe regime and study its multiwavelength (MW) photon spectrum. We develop a generic analytical framework and use numerical calculations as a backup to the analytical predictions. We find that the escaping radiation from a source in the Compton catastrophe regime bears some unique features. The MW photon spectrum is a broken power law with a break at ∼mec2 due to the onset of the Klein-Nishina suppression. The spectral index below the break energy depends on the electron and magnetic compactnesses logarithmically, while it is independent of the electron power-law index (s). The maximum radiating power emerges typically in the γ-ray regime, at energies ∼mec2 (∼γmax mec2) for s > 2 (s ≲ 2), where γmax is the maximum Lorentz factor of the injected electron distribution. We apply the principles of the inverse Compton catastrophe to blazars and γ-ray bursts using the analytical framework we developed, and show how these can be used to impose robust constraints on the source parameters.
2014
Petropoulou M. The role of hadronic cascades in GRB models of efficient neutrino production. [Internet]. 2014;442:3026 - 3036. WebsiteAbstract
We investigate the effects of hadronic cascades on the gamma-ray burst (GRB) prompt emission spectra in scenarios of efficient neutrino production. By assuming a fiducial GRB spectrum and a power-law proton distribution extending to ultrahigh energies, we calculate the proton cooling rate and the neutrino emission produced through photopion processes. For this, we employ a numerical code that follows the formation of the hadronic cascade by taking into account non-linear feedback effects, such as the evolution of the target photon field itself due to the contribution of secondary particles. We show that in cases of efficient proton cooling and subsequently efficient high-energy neutrino production, the emission from the hadronic cascade distorts and may even dominate the GRB spectrum. Taking this into account, we constrain the allowable values of the ratio ηp = Lp/Lγ, where Lp and Lγ are the isotropic equivalent proton and prompt gamma-ray luminosities. For the highest value of ηp that does not lead to the dominance of the cascading emission, we then calculate the maximum neutrino luminosity from a single burst and show that it ranges between (0.01-0.6)Lp and (0.5-1.4)Lγ for various parameter sets. We discuss possible implications of other parameters, such as the magnetic field strength and the shape of the initial gamma-ray spectrum, on our results. Finally, we compare the upper limit on ηp derived here with various studies in the field, and we point out the necessity of a self-consistent treatment of the hadronic emission in order to avoid erroneously high neutrino fluxes from GRB models.
Petropoulou M, Dimitrakoudis S, Mastichiadis A. Neutrino and Uhecr Spectra from MRK 421. In: Vol. 28. ; 2014. pp. 1460206. WebsiteAbstract
We present the neutrino and UHECR spectra obtained from a detailed fitting of the spectral energy distribution (SED) of Mrk 421 (March 2001) using two variations of the leptohadronic model. In particular, while the low-energy component (optical to X-rays) of the SED is fitted by synchrotron emission of primary electrons in both models, the high-energy one (GeV-TeV gamma-rays) is synchrotron emission attributed either to ultra-high energy protons (LHs model) or to secondary electrons produced by the decay of charged pions (LHπ model). In the LHπ case we find that the produced neutrino spectra are sharply peaked at Eν 30 PeV with a peak flux slightly below the IC-40 sensitivity limit for Mrk 421. In the LHs model, on the other hand, the neutrino spectra fall well outside the PeV energy range, but the calculated E 30 EeV — UHECR flux at earth is close to that observed by HiresI, Telescope Array and Pierre Augere experiments.
Petropoulou M, Lefa E, Dimitrakoudis S, Mastichiadis A. One-zone synchrotron self-Compton model for the core emission of Centaurus A revisited. [Internet]. 2014;562:A12. WebsiteAbstract
Aims: We investigate the role of the second synchrotron self-Compton (SSC) photon generation to the multiwavelength emission from the compact regions of sources that are characterized as misaligned blazars. For this, we focus on the nearest high-energy emitting radio galaxy Centaurus A and we revisit the one-zone SSC model for its core emission. Methods: We have calculated analytically the peak luminosities of the first and second SSC components by first deriving the steady-state electron distribution in the presence of synchrotron and SSC cooling, and then by using appropriate expressions for the positions of the spectral peaks. We have also tested our analytical results against those derived from a numerical code where the full emissivities and cross-sections were used. Results: We show that the one-zone SSC model cannot account for the core emission of Centaurus A above a few GeV, where the peak of the second SSC component appears. We thus propose an alternative explanation for the origin of the high-energy (≳0.4 GeV) and TeV emission, where these are attributed to the radiation emitted by a relativistic proton component through photohadronic interactions with the photons produced by the primary leptonic component. We show that the required proton luminosities are not extremely high, i.e. ~1043 erg/s, provided that the injection spectra are modelled by a power law with a high value of the lower energy cutoff. Finally, we find that the contribution of the core emitting region of Cen A to the observed neutrino and ultra-high-energy cosmic-ray fluxes is negligible.
Dimitrakoudis S, Petropoulou M, Mastichiadis A. Self-consistent neutrino and UHE cosmic ray spectra from Mrk 421. [Internet]. 2014;54:61 - 66. WebsiteAbstract
We examine the neutrino and cosmic ray spectra resulting from two models of fitting the spectral energy distribution (SED) of the blazar Mrk 421 using a self-consistent leptohadronic code. The γ -ray emission is attributed to either synchrotron radiation of ultra-high energy protons (LHs model) or to synchrotron radiation from electrons that result from photopion interactions of lower energy protons (LH π model). Although both models succeed in fitting satisfactorily the SED, the parameter values that they use result in significantly different neutrino and cosmic-ray spectra. For the LH π model, which requires high proton energy density, we find that the neutrino spectrum peaks at an energy Eν,peak = 3.3 PeV which falls well within the energy range of recent neutrino observations. While at the same time its peak flux is just under the sensitivity limit of IC-40 observations, it cannot produce ultra-high energy cosmic rays. In the LHs model, on the other hand, neutrinos are far from being detectable because of their low flux and peak energy at Eν,peak ≃ 100 PeV. However, the propagation of protons produced by the decay of escaping neutrons results in an ultra-high energy cosmic ray flux close to that observed by Pierre Augere, HiRes and Telescope Array at energies Ep ≃ 30 EeV.
Petropoulou M, Dimitrakoudis S, Mastichiadis A, Giannios D. Hadronic supercriticality as a trigger for γ-ray burst emission. [Internet]. 2014;444:2186 - 2199. WebsiteAbstract
We explore a one-zone hadronic model that may be able to reproduce γ-ray burst (GRB) prompt emission with a minimum of free parameters. Assuming only that GRBs are efficient high-energy proton accelerators and without the presence of an ab initio photon field, we investigate the conditions under which the system becomes supercritical, i.e. there is a fast, non-linear transfer of energy from protons to secondary particles initiated by the spontaneous quenching of proton-produced γ-rays. We first show analytically that the transition to supercriticality occurs whenever the proton injection compactness exceeds a critical value, which favours high proton injection luminosities and a wide range of bulk Lorentz factors. The properties of supercriticality are then studied with a time-dependent numerical code that solves concurrently the coupled equations of proton, photon, electron, neutron and neutrino distributions. For conditions that drive the system deep into the supercriticality, we find that the photon spectra obtain a Band-like shape due to Comptonization by cooled pairs and that the energy transfer efficiency from protons to γ-rays and neutrinos is high reaching ∼0.3. Although some questions concerning its full adaptability to the GRB prompt emission remain open, supercriticality is found to be a promising process in that regard.
Petropoulou M, Giannios D, Dimitrakoudis S. Implications of a PeV neutrino spectral cut-off in gamma-ray burst models. [Internet]. 2014;445:570 - 580. WebsiteAbstract
The recent discovery of extragalactic PeV neutrinos opens a new window to the exploration of cosmic ray accelerators. The observed PeV neutrino flux is close to the Waxman-Bahcall upper bound implying that gamma-ray bursts (GRBs) may be the source of ultrahigh energy cosmic rays (UHECRs). Starting with the assumption of the GRB-UHECR connection, we show using both analytical estimates and numerical simulations that the observed neutrinos can originate at the jet as a result of photopion interactions with the following implications: the neutrino spectra are predicted to have a cut-off at energy ≲10 PeV; the dissipation responsible for the GRB emission and cosmic ray acceleration takes place at distances rdiss ≃ 3 × 1011-3 × 1013 cm from the central engine; the Thomson optical depth at the dissipation region is τT ∼ 1; the jet carries a substantial fraction of its energy in the form of Poynting flux at the dissipation region, and has a Lorentz factor Γ ≃ 100-500. The non-detection of PeV neutrinos coincident with GRBs will indicate that GRBs are either poor cosmic accelerators or the dissipation takes place at small optical depths in the jet.
Petropoulou M. Time-dependent modelling of PKS 2155-304 in a low state. [Internet]. 2014;571:A83. WebsiteAbstract
Aims: We apply both leptonic and leptohadronic emission scenarios for modelling the multiwavelength photon spectra and the observed variability in the optical, X-ray, and TeV gamma-ray energy bands of blazar PKS 2155-304 while being in a low state between 25 August and 6 September 2008. Methods: We consider three emission models, namely a one-component synchrotron self-Compton model (1-SSC), a one-zone proton synchrotron model (LHs), and a two-component SSC model (2-SSC). Only in the first scenario can the emission from the optical up to very high-energy (VHE) gamma-rays be attributed to a single particle population from one emission region. In the LHs model, the low-energy and high-energy bumps of the spectral energy distribution (SED) are the result of electron and proton synchrotron radiation, respectively, i.e. two different particle populations are required. In the 2-SSC model, the emission from one component dominates in the optical and gamma-ray energy bands, while the other one contributes only to the X-ray flux. Using a time-dependent numerical code that solves the kinetic equations for each particle species, we derived, in all cases, acceptable fits to the time-averaged SED. By imposing variations to one (or more) model parameters according to observed variability pattern in one (or more) frequencies we calculated the respective lightcurves and compared them with the observations. Results: We show that the 1-SSC model cannot account for the anticorrelation observed between the X-rays and VHE gamma-rays, although it can explain the time-averaged SED. The anticorrelation can be more naturally explained by the two-component emission models. Both of them reproduce satisfactorily the optical, X-ray, and TeV variability but at the cost of additional free parameters, which from four in the 2-SSC model increase to six in the LHs model. Although the results of our time-resolved analysis do not favour one of the aforementioned models, they suggest that a two-component scenario is more adequate for the emission of PKS 2155-304 in the low state of 2008, which agrees with a recent independent analysis. This suggests that the quiescent blazar radiation might result from a superposition of the radiation from different components, while a flare might still be the result of a single component.
Petropoulou M. High-Energy Signatures from Leptohadronic Interactions in GRB Models. In: ; 2014. pp. 12. WebsiteAbstract
Gamma-ray bursts (GRB) have been long considered to be the sources of ultra high energy cosmic rays. If GRB jets are, indeed, sites of proton acceleration at high energies, then photohadronic processes, i.e. interactions between protons and photons, become relevant. In this talk, I will discuss some of their consequences for GRB models. First, I will present how we can constrain the physical conditions of the GRB emitting region by using indirect information from the copious neutrino emission that is naturally produced via photohadronic interactions on an ad-hoc Band photon spectrum. Second, I will present a model for the formation of Band-like photon spectra from first principles. This has been built on a recently discovered radiative instability, known as "spontaneous photon quenching." I will show that for a wide parameter range the instability sets in and establishes an efficient energy transfer from protons to secondaries produced through photohadronic interactions. It is then the interplay between photons and secondary electron-positron pairs, through purely leptonic processes, that actually determines the shape of the gamma-ray spectrum at steady-state.
2013
Mastichiadis A, Petropoulou M, Dimitrakoudis S. Time dependent photon and neutrino emission from Mkr 421 in the context of the one-zone leptohadronic model. In: Vol. 61. ; 2013. pp. 05005. WebsiteAbstract
We apply a recently developed time-dependent one-zone leptohadronic model to study the emission of the blazar Mrk 421. Both processes involving proton-photon interactions, i.e. photopair (Bethe-Heitler) and photopion, have been modeled in great detail using the results of Monte Carlo simulations, like the SOPHIA event generator, in a self-consistent scheme that couples energy losses and secondary injection. We find that TeV gamma-rays can be attributed to synchrotron radiation either from relativistic protons or, alternatively, from secondary leptons produced via photohadronic processes. We also study the variability patterns that each scenario predicts and we find that while the former is more energetically favored, it is the latter that produces, in a more natural way, the usual quadratic behavior between X-rays and TeV gamma-rays. We also use the obtained SEDs to calculate in detail the expected neutron and neutrino fluxes that each model predicts.
Petropoulou M, Mastichiadis A. Time-dependent modelling of PKS 2155-304 in a low state: one- or two-zone emission modelling?. In: Vol. 61. ; 2013. pp. 05013. WebsiteAbstract
One-zone radiation models have been widely used in modelling the steady-state multiwavelength (MW) spectra of blazars, having as main goal the determination of the physical conditions in the emitting region, such as the magnetic field strength, the species of radiating particles etc. Then, the results from onezone stationary modelling are often used as a stepping stone for studying flaring events. Here we show that the application of steady-state one-zone models on intrinsic variable sources, even when these are in a low state, can be misleading. Although the one-zone SSC and proton synchrotron models succeed in fitting the time-averaged MWspectrum, they cannot easily (or at all) reproduce the small amplitude multifrequency variability.We show that a two-component leptonic model addresses both spectral and temporal observations more successfully, albeit at the expense of more free parameters.
Gazeas K, Petropoulou M, Mastichiadis A. Continuous optical monitoring of the highly active blazar Mrk421. In: ; 2013. pp. 30 - 30. WebsiteAbstract
We present the recent photometric monitoring of blazar Mrk421, obtained from the Gerostathopouleio Observatoty at University of Athens. Follow-up observations have been performed on this source after a highly energetic flare which occurred on 13 April, 2013. The flare was observed in X- rays by Nustar & Swift and in GeV - TeV gamma-rays by the Fermi satellite and MAGIC/VERITAS telescopes respectively. Continuous photometric monitoring in the optical BVRI bands during 3 months after the flaring activity reveals a quasi-periodic light variation. This is one of the few times that Mrk 421 was observed for such a long period without large observational gaps. We perform Fourier analysis of the almost uninterrupted 3-months-long dataset in order to get insight on the characteristic timescales of the system. We discuss also possible origins of the optical variability by performing cross-correlation analysis of the optical and of the simultaneous X-ray emission detected with XRT onboard the SWIFT orbital satellite.
Petropoulou M. A mechanism for producing intrinsic broken power-law gamma-ray spectra in compact sources. In: ; 2013. pp. 23 - 23. WebsiteAbstract
We study a mechanism for producing intrinsic broken power-law gamma-ray spectra in compact sources. This is based on the principles of automatic photon quenching, according to which, gammarays are being absorbed on spontaneously produced soft photons, whenever the injected luminosity in gamma-rays lies above a certain critical value. We derive an analytical expression for the critical gamma-ray compactness in the case of power-law injection. For the case where automatic photon quenching is relevant, we calculate analytically the emergent steady-state gamma-ray spectra. We show that a spontaneously quenched power-law gamma-ray spectrum obtains a photon index 3√/2, where √ is the photon index of the power-law at injection. Thus, large spectral breaks of the gammaray photon spectrum, e.g. ƒ√>1, can be obtained by this mechanism. We also discuss additional features of this mechanism that can be tested observationally. Finally, we fit the multiwavelength spectrum of a newly discovered blazar (PKS 0447-439) by using such parameters, as to explain the break in the gamma-ray spectrum by means of spontaneous photon quenching.
Mastichiadis A, Petropoulou M, Dimitrakoudis S. Mrk 421 as a case study for TeV and X-ray variability in leptohadronic models. [Internet]. 2013;434:2684 - 2695. WebsiteAbstract
We investigate the origin of high-energy emission in blazars within the context of the leptohadronic one-zone model. We find that γ-ray emission can be attributed to synchrotron radiation either from protons or from secondary leptons produced via photohadronic processes. These possibilities imply differences not only in the spectral energy distribution (SED) but also in the variability signatures, especially in the X- and γ-ray regime. Thus, the temporal behaviour of each leptohadronic scenario can be used to probe the particle population responsible for the high-energy emission as it can give extra information not available by spectral fits. In this work, we apply these ideas to the non-thermal emission of Mrk 421, which is one of the best monitored TeV blazars. We focus on the observations of 2001 March, since during that period Mrk 421 showed multiple flares that have been observed in detail both in X-rays and γ-rays. First, we obtain pre-flaring fits to the SED using the different types of leptohadronic scenarios. Then, we introduce random-walk-type, small-amplitude variations on the injection compactness or on the maximum energy of radiating particles and follow the subsequent response of the radiated photon spectrum. For each leptohadronic scenario, we calculate the X-ray and γ-ray fluxes and investigate their possible correlation. Whenever the input' variations lead, apart from flux variability, also to spectral variability, we present the resulting relations between the spectral index and the flux, both in X-rays and γ-rays. We find that proton synchrotron models are favoured energetically but require fine tuning between electron and proton parameters to reproduce the observed quadratic behaviour between X-rays and TeV γ-rays. On the other hand, models based on pion decay can reproduce this behaviour in a much more natural way.
Petropoulou M, Arfani D, Mastichiadis A. Spontaneously quenched γ-ray spectra from compact sources. [Internet]. 2013;557:A48. WebsiteAbstract
Aims: We have studied a mechanism for producing intrinsic broken power-law γ-ray spectra in compact sources. This is based on the principles of automatic photon quenching, according to which γ-rays are being absorbed on spontaneously produced soft photons whenever the injected luminosity in γ-rays lies above a certain critical value. Methods: We derived an analytical expression for the critical γ-ray compactness in the case of power-law injection. For the case where automatic photon quenching is relevant, we calculated analytically the emergent steady-state γ-ray spectra. We also performed numerical calculations in order to back up our analytical results. Results: We show that a spontaneously quenched power-law γ-ray spectrum obtains a photon index 3Γ/2, where Γ is the photon index of the power-law at injection. Thus, large spectral breaks of the γ-ray photon spectrum, e.g. ∆Γ ≳ 1, can be obtained by this mechanism. We also discuss additional features of this mechanism that can be tested observationally. Finally, we fit the multiwavelength spectrum of a newly discovered blazar (PKS 0447-439) by using such parameters to explain the break in the γ-ray spectrum by means of spontaneous photon quenching, under the assumption that its redshift lies in the range 0.1 < z < 0.24.
2012
Petropoulou M, Mastichiadis A. An estimation method for the minimum Doppler factor and energy content of quasar 3C 279. In: Vol. 1505. ; 2012. pp. 643 - 646. WebsiteAbstract
In the present work we propose an estimation method for the minimum Doppler factor and energy content of the γ-ray emitting region of quasar 3C 279, using a standard proton synchrotron blazar model and the principles of automatic photon quenching. This estimation method can be regarded as an extension of the one for estimating the equipartition magnetic field. In our case the leptonic synchrotron component is replaced by the proton synchrotron emission and the radio by the VHE γ-ray observations.
Petropoulou M, Mastichiadis A. On proton synchrotron blazar models: the case of quasar 3C 279. [Internet]. 2012;426:462 - 472. WebsiteAbstract
In this work we propose an innovative estimation method for the minimum Doppler factor and energy content of the γ-ray emitting region of quasar 3C 279, using a standard proton synchrotron blazar model and the principles of automatic photon quenching. The latter becomes relevant for high enough magnetic fields and results in spontaneous annihilation of γ-rays. The absorbed energy is then redistributed into electron-positron pairs and soft radiation. We show that as quenching sets an upper value for the source rest-frame γ-ray luminosity, one has, by necessity, to resort to Doppler factors that lie above a certain value in order to explain the TeV observations. The existence of this lower limit for the Doppler factor also has implications on the energetics of the emitting region. In this aspect, the proposed method can be regarded as an extension of the widely used method for estimating the equipartition magnetic field using radio observations. In our case, the leptonic synchrotron component is replaced by the proton synchrotron emission and the radio by the very high energy γ-ray observations. We show specifically that one can model the TeV observations by using parameter values that minimize both the energy density and the jet power at the cost of high values of the Doppler factor. On the other hand, the modelling can also be done by using the minimum possible Doppler factor; this, however, leads to a particle-dominated region and high jet power for a wide range of magnetic field values. Despite the fact that we have focused on the case of 3C 279, our analysis can be of relevance to all TeV blazars favouring hadronic modelling that have, moreover, simultaneous X-ray observations.
Petropoulou M, Mastichiadis A. Temporal signatures of leptohadronic feedback mechanisms in compact sources. [Internet]. 2012;421:2325 - 2341. WebsiteAbstract
The hadronic model of active galactic nuclei and other compact high-energy astrophysical sources assumes that ultra-relativistic protons, electron-positron pairs and photons interact via various hadronic and electromagnetic processes inside a magnetized volume, producing the multiwavelength spectra observed from these sources. A less studied property of such systems is that they can exhibit a variety of temporal behaviours due to the operation of different feedback mechanisms. We investigate the effects of one possible feedback loop, where γ-rays produced by photopion processes are being quenched whenever their compactness increases above a critical level. This causes a spontaneous creation of soft photons in the system that result in further proton cooling and more production of γ-rays, thus making the loop operate. We perform an analytical study of a simplified set of equations describing the system, in order to investigate the connection of its temporal behaviour with key physical parameters. We also perform numerical integration of the full set of kinetic equations verifying not only our analytical results but also those of previous numerical studies. We find that once the system becomes 'supercritical', it can exhibit either a periodic behaviour or a damped oscillatory one leading to a steady state. We briefly point out possible implications of such a supercriticality on the parameter values used in active galactic nuclei spectral modelling, through an indicative fitting of the VHE emission of blazar 3C 279.
Petropoulou M, Mastichiadis A, Piran T. Afterglow emission in the context of an 'one-zone' radiation-acceleration model. In: ; 2012. pp. 67. Website
Petropoulou M, Mastichiadis A. Automatic Quenching of γ-RAY Emission in Compact Astrophysical Sources. In: Vol. 8. ; 2012. pp. 384 - 387. WebsiteAbstract
We investigate automatic γ-ray photon quenching in compact non-thermal sources. This is an auto-regulating network of processes that consists of photon-photon absorption and synchrotron emission of the produced e- e+ pairs and operates non linearly whenever the γ-ray luminosity exceeds a critical value. We present expressions for this quantity and discuss our results.
Petropoulou M. Signatures of particle acceleration on Gamma Ray Burst afterglow light curves. In: ; 2012. pp. 17 - 17. WebsiteAbstract
We investigate the behavior of the X-ray lightcurves in the afterglow phase of Gamma Ray Bursts (GRB), after taking into account the maximum electron Lorentz factor (gamma_max) as an additional parameter of the problem. First, we treat gamma_max as a free parameter and we examine the lightcurves that one obtains for different values of the ratio gamma_max/gamma_min, where gamma_min is the minimum electron energy. We find that the lightcurves depend strongly on this ratio showing a variety of morphologies, with some having a strong resemblance to the observations. As a next step, we introduce particle acceleration and calculate gamma_max in a self-consistent way by balancing the energy losses with the energy gains of the accelerating electrons. The physical picture corresponds to particles injected at low energies and accelerated in the downstream region of the external GRB shock wave. We simulate that by introducing an acceleration term in the equation that describes the evolution of the electron distribution. We show some first results of the radiated synchrotron photon spectra obtained at various radii of the blast wave. Finally, we discuss possible implications of such 'one-zone' acceleration models for GRB afterglows.
Dimitrakoudis S, Petropoulou M, Mastichiadis A. The Time-Dependent One-Zone Hadronic Model: First Principles. In: Vol. 8. ; 2012. pp. 19 - 24. WebsiteAbstract
We present some results on the radiative signatures of the one zone hadronic model. For this we have solved five spatially averaged, time-dependent coupled kinetic equations which describe the evolution of relativistic protons, electrons, photons, neutrons and neutrinos in a spherical volume containing a magnetic field. Protons are injected and lose energy by synchrotron, photopair and photopion production. We model photopair and photopion using the results of relevant MC codes, like the SOPHIA code in the case of photopion, which give accurate description for the injection of secondaries which then become source functions in their respective equations. This approach allows us to calculate the expected photon and neutrino spectra simultaneously in addition to examining questions like the efficiency and the temporal behaviour of the hadronic models.
2011
Petropoulou M, Mastichiadis A. Implications of automatic photon quenching on compact gamma-ray sources. [Internet]. 2011;532:A11. WebsiteAbstract
Aims: We investigate photon quenching in compact non-thermal sources. This involves photon-photon annihilation and lepton synchrotron radiation in a network that can become non-linear. As a result the γ-ray luminosity of a source cannot exceed a critical limit that depends only on the radius of the source and on the magnetic field. Methods: We perform analytic and numerical calculations that verify previous results and extend them so that the basic properties of photon quenching are investigated. Results: We apply the above to the 2006 TeV observations of quasar 3C 279 and obtain the parameter space of allowed values for the radius of the emitting source, its magnetic field strength and the Doppler factor of the flow. We argue that the TeV observations favour either a modest Doppler factor and a low magnetic field or a high Doppler factor and a high magnetic field.
Petropoulou M, Mastichiadis A, Piran T. Effects of a low electron distribution cutoff on multiwavelength spectra and light curves of GRB afterglows. [Internet]. 2011;531:A76. WebsiteAbstract
Aims: We investigate the behavior of the frequency-centered light curves expected within the standard model of gamma ray bursts, allowing the maximum electron energy (γmax) to be a free parameter that may take low values. Methods: We solve the spatially averaged kinetic equations that describe the simultaneous evolution of particles and photons, obtaining the multi-wavelength spectra as a function of time. From these we construct the frequency-centered light curves with an emphasis on the X-ray and optical bands. Results: We show that in cases where γmax takes low values, the produced X-ray light curves show a plateau as the synchrotron component gives its place to the synchrotron self-Compton one in the X-ray band.
2010
Petropoulou M, Mastichiadis A. On the Synchrotron and SSC Emission of GRB Afterglows — Some Analytical Results. In: Vol. 424. ; 2010. pp. 314. WebsiteAbstract
We present analytical and numerical results of the electron radiation spectrum during the GRB afterglow phase as produced by particle acceleration in an external shockwave.
Petropoulou M, Mastichiadis A, Piran T. Effects of the upper cutoff of the electron distribution on the light curves of GRB afterglows. In: ; 2010. pp. 98. Website
Petropoulou M, Mastichiadis A. X-ray plateaus in the context of the one-zone SSC model for GRB afterglows. In: Vol. 1279. ; 2010. pp. 379 - 381. WebsiteAbstract
We investigate the impact that the upper cutoff of the electron distribution has on the multiwavelength GRB afterglow spectra and on the corresponding X-ray light curves. We show under which conditions X-ray light curves with a plateau phase can be produced in this picture.
2009
Petropoulou M, Mastichiadis A. On the multiwavelength emission from gamma ray burst afterglows. [Internet]. 2009;507:599 - 610. WebsiteAbstract
Aims: Drawing an analogy with active galactic nuclei, we investigate the one-zone synchrotron self-compton (SSC) model of gamma ray bursts (GRB) afterglows in the presence of electron injection and cooling both by synchrotron and SSC losses. Methods: We solve the spatially averaged kinetic equations which describe the simultaneous evolution of particles and photons, obtaining the multi-wavelength spectrum as a function of time. We back up our numerical calculations with analytical solutions of the equations using various profiles of the magnetic field evolution under certain simplifying assumptions. Results: We apply the model to the afterglow evolution of GRBs in a uniform density environment and examine the impact various parameters have on the multiwavelength spectra. We find that in cases where the electron injection and/or the ambient density is high, the losses are dominated by SSC and the solutions depart significantly from the ones derived in the synchrotron standard cases.