# Publications

2021
Mastichiadis A, Petropoulou M. Hadronic X-Ray Flares from Blazars. [Internet]. 2021;906:131. WebsiteAbstract
2020
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. 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.
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. 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.
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.
2019
Petropoulou M, Yuan Y, Chen AY, Mastichiadis A. Inverse Compton Cascades in Pair-producing Gaps: Effects of Triplet Pair Production. [Internet]. 2019;883. 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.
Boula S, Kazanas D, Mastichiadis A. Accretion disc MHD winds and blazar classification. [Internet]. 2019;482:L80 - L84. WebsiteAbstract
The Fermi Gamma-Ray Space Telescope observations of blazars show a strong correlation between the spectral index of their γ-ray spectra and their synchrotron peak frequency ν _{pk}^{syn}; additionally, the rate of Compton dominance of these sources also seems to be a function of ν _{pk}^{syn}. In this work, we adopt the assumption that the non-thermal emission of blazars is primarily due to radiation by a population of Fermi-accelerated electrons in a relativistic outflow (jet) along the symmetry axis of the blazar's accretion disc. Furthermore, we assume that the Compton component is related to an external photon field of photons, which are scattered from particles of the magnetohydrodynamic wind emanating from the accretion disc. Our results reproduce well the aforementioned basic observational trends of blazar classification by varying just one parameter, namely the mass accretion rate on to the central black hole.
2018
Petropoulou M, Mastichiadis A. Patterns of variability in supercritical hadronic systems. [Internet]. 2018;477. 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.
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. 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.
Boula S, Petropoulou M, Mastichiadis A. On the Connection of Radio and γ-Ray Emission in Blazars. [Internet]. 2018;7. 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.
2017
Petropoulou M, Nalewajko K, Hayashida M, Mastichiadis A. A hadronic minute-scale GeV flare from quasar 3C 279?. [Internet]. 2017;467. 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.
2016
Mastichiadis A. Consequences of Proton Acceleration in Blazar Jets. [Internet]. 2016;4:59. WebsiteAbstract
Hadronic models of blazar emission constitute an interesting alternative to the more popular leptonic ones. Using the BL Lac object Mrk 421 as a characteristic example, we present two distinct ways of modeling the spectral energy distribution of blazars in the hadronic context, and we discuss the predictions of each 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.
2015
Petropoulou M, Mastichiadis A. Bethe-Heitler emission in BL Lacs: filling the gap between X-rays and γ-rays. [Internet]. 2015;447. 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, Padovani P, Mastichiadis A, Resconi E. Photohadronic origin of γ -ray BL Lac emission: implications for IceCube neutrinos. [Internet]. 2015;448. 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, Piran T, Mastichiadis A. Spectral signatures of compact sources in the inverse Compton catastrophe limit. [Internet]. 2015;452. 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.
Kazanas D, Racusin JL, Sultana J, Mastichiadis A. The Statistics of BAT-To-XRT Flux Ratio in GRBs: Evidence for a Characteristic Value and Its Implications. [Internet]. 2015;802:83. WebsiteAbstract
We present the statistics of the ratio, R, between the prompt and afterglow “plateau” fluxes of gamma-ray bursts (GRBs). We define this as the ratio of the mean prompt energy flux in Swift BAT and the Swift XRT one, immediately following the steep transition between these two states and the beginning of the afterglow stage referred to as the “plateau”. Like the distribution of many other GRB observables, the histogram of R is log-normal with maximum at a value {{R}m}≃ 2000, FWHM of about two decades, and with the entire distribution spanning about five decades in the value of R. We note that the peak of the distribution is close to the proton-to-electron mass ratio ({{R}m}≃ {{m}p}/{{m}e}=1836), as proposed to be the case in an earlier publication, on the basis of a specific model of the GRB dissipation process. It therefore appears that, in addition to the values of the energy of peak luminosity {{E}pk}∼ {{m}e}{{c}2}, GRBs present us with one more quantity with an apparent characteristic value. The fact that the values of both these quantities ({{E}pk} and R) are consistent with the same specific model invoked to account for the efficient conversion of their relativistic proton energies to electrons argues favorably for its underlying assumptions.
2014
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. 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. 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. 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.
2013
Acharya BS, Actis M, Aghajani T, Agnetta G, Aguilar J, Aharonian F, Ajello M, Akhperjanian A, Alcubierre M, Aleksić J, et al. Introducing the CTA concept. [Internet]. 2013;43. WebsiteAbstract
The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project.
Mastichiadis A, Petropoulou M, Dimitrakoudis S. Mrk 421 as a case study for TeV and X-ray variability in leptohadronic models. [Internet]. 2013;434. WebsiteAbstract
Sultana J, Kazanas D, Mastichiadis A. The Supercritical Pile Gamma-Ray Burst Model: The GRB Afterglow Steep Decline and Plateau Phase. [Internet]. 2013;779:16. WebsiteAbstract
We present a process that accounts for the steep decline and plateau phase of the Swift X-Ray Telescope (XRT) light curves, vexing features of gamma-ray burst (GRB) phenomenology. This process is an integral part of the "supercritical pile" GRB model, proposed a few years ago to account for the conversion of the GRB kinetic energy into radiation with a spectral peak at E pk ~ mec 2. We compute the evolution of the relativistic blast wave (RBW) Lorentz factor Γ to show that the radiation-reaction force due to the GRB emission can produce an abrupt, small (~25%) decrease in Γ at a radius that is smaller (depending on conditions) than the deceleration radius RD . Because of this reduction, the kinematic criticality criterion of the "supercritical pile" is no longer fulfilled. Transfer of the proton energy into electrons ceases and the GRB enters abruptly the afterglow phase at a luminosity smaller by ~mp /me than that of the prompt emission. If the radius at which this slow-down occurs is significantly smaller than RD , the RBW internal energy continues to drive the RBW expansion at a constant (new) Γ and its X-ray luminosity remains constant until RD is reached, at which point it resumes its more conventional decay, thereby completing the "unexpected" XRT light curve phase. If this transition occurs at R ~= RD , the steep decline is followed by a flux decrease instead of a "plateau," consistent with the conventional afterglow declines. Besides providing an account of these peculiarities, the model suggests that the afterglow phase may in fact begin before the RBW reaches R ~= RD , thus providing novel insights into GRB phenomenology.
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. ΔΓ ≳ 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. Temporal signatures of leptohadronic feedback mechanisms in compact sources. [Internet]. 2012;421. 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. On proton synchrotron blazar models: the case of quasar 3C 279. [Internet]. 2012;426. 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.
Dimitrakoudis S, Mastichiadis A, Protheroe RJ, Reimer A. The time-dependent one-zone hadronic model. First principles. [Internet]. 2012;546:A120. WebsiteAbstract
We present a time-dependent approach to the one-zone hadronic model in the case where the photon spectrum is produced by ultrarelativistic protons interacting with soft photons that are produced from protons and low magnetic fields. Assuming that protons are injected at a certain rate in a homogeneous spherical volume containing a magnetic field, the evolution of the system can be described by five coupled kinetic equations, for protons, electrons, photons, neutrons, and neutrinos. Photopair and photopion interactions are modelled using the results of Monte-Carlo simulations and, in particular, from the SOPHIA code for the latter. The coupling of energy losses and injection introduces a self-consistency in our approach and allows the study of the comparative relevancy of processes at various conditions, the efficiency of the conversion of proton luminosity to radiation, the resulting neutrino spectra, and the effects of time variability on proton injection, among other topics. We present some characteristic examples of the temporal behaviour of the system and show that this can be very different from the one exhibited by leptonic models. Furthermore, we argue that, contrary to the wide-held belief, there are parameter regimes where the hadronic models can become quite efficient. However, to keep the free parameters at a minimum and facilitate an in-depth study of the system, we have only concentrated on the case where protons are injected; i.e., we did not consider the effects of a co-accelerated leptonic component.
2011
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. 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.
Petropoulou M, Mastichiadis A. Implications of automatic photon quenching on compact gamma-ray sources. [Internet]. 2011;532. 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.
Actis M, Agnetta G, Aharonian F, Akhperjanian A, Aleksić J, Aliu E, Allan D, Allekotte I, Antico F, Antonelli LA, et al. Design concepts for the Cherenkov Telescope Array CTA: an advanced facility for ground-based high-energy gamma-ray astronomy. [Internet]. 2011;32. WebsiteAbstract
Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA.
Moraitis K, Mastichiadis A. X-ray variability patterns in blazars. [Internet]. 2011;525:A40. WebsiteAbstract
Aims: We study the expected variability patterns of blazars within the two-zone acceleration model, putting special emphasis on flare shapes and spectral lags. Methods: We semi-analytically solve the kinetic equations that describe the particle evolution in the acceleration and radiation zone. We then perturb the solutions by introducing Lorentzian variations in its key parameters and examine the flaring behavior of the system. We apply the above to the X-ray observations of blazar 1ES 1218+304, which exhibited a hard lag behavior during a flaring episode and discuss possibilities of producing it within the context of our model. Results: The steady-state radio to X-rays emission of 1ES 1218+304 can be reproduced with parameters that lie well within the ones generally accepted from blazar modeling. Additionally, we find that the best way to explain its flaring behavior is by varying the rate of particles injected in the acceleration zone.
2009
Petropoulou M, Mastichiadis A. On the multiwavelength emission from gamma ray burst afterglows. [Internet]. 2009;507. 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.
Dimitrakoudis S, Mastichiadis A, Geranios A. Obtaining the diffusion coefficient for cosmic ray propagation in the Galactic Centre Ridge through time-dependent simulations of their γ-ray emission. [Internet]. 2009;31:13 - 18. WebsiteAbstract
Recent observations by the H.E.S.S. collaboration of the Galactic Centre region have revealed what appears to be γ-ray emission from the decay of pions produced by interactions of recently accelerated cosmic rays with local molecular hydrogen clouds. Synthesizing a 3D hydrogen cloud map from the available data and assuming a diffusion coefficient of the form κ(E) = κ0(E/E0)δ, we performed Monte Carlo simulations of cosmic ray diffusion for various propagation times and values of κ0 and δ. By fitting the model γ-ray spectra to the observed one we were able to infer the value of the diffusion coefficient in that environment (κ = 3.0 ± 0.2 kpc2 Myr-1 for E = 1012.5 eV and for total propagation time 104 yr) as well as the source spectrum (2.1 ⩽ γ ⩽ 2.3). Also, we found that proton losses can be substantial, which justifies our approach to the problem.
Mastichiadis A, Kazanas D. The Supercritical Pile Gamma-Ray Burst Model: The Prompt to Afterglow Evolution. [Internet]. 2009;694:L54 - L58. WebsiteAbstract
The "Supercritical Pile" is a very economical gamma-ray burst (GRB) model that provides for the efficient conversion of the energy stored in the protons of a relativistic blast wave (RBW) into radiation and at the same time produces—in the prompt GRB phase, even in the absence of any particle acceleration—a spectral peak at energy ~1MeV. We extend this model to include the evolution of the RBW Lorentz factor Γ and thus follow its spectral and temporal features into the early GRB afterglow stage. One of the novel features of the present treatment is the inclusion of the feedback of the GRB produced radiation on the evolution of Γ with radius. This feedback and the presence of kinematic and dynamic thresholds in the model are sources of potentially very rich time evolution which we have began to explore. In particular, one can this way obtain afterglow light curves with steep decays followed by the more conventional flatter afterglow slopes, while at the same time preserving the desirable features of the model, i.e., the well-defined relativistic electron source and radiative processes that produce the proper peak in the νF ν spectra. In this Letter, we present the results of a specific set of parameters of this model with emphasis on the multiwavelength prompt emission and transition to the early afterglow.
2008
Mastichiadis A, Moraitis K. On the rapid TeV flaring activity of Markarian 501. [Internet]. 2008;491:L37 - L40. WebsiteAbstract
Aims: We investigate the one-zone SSC model of TeV blazars in the presence of electron acceleration. In this picture, electrons achieve their maximum energy as the acceleration saturates due to a combination of synchrotron and inverse Compton scattering losses. Methods: We solve the spatially averaged kinetic equations that describe the simultaneous evolution of particles and photons, obtaining the multiwavelength spectrum as a function of time. Results: We apply the model to the rapid flare of Mrk 501 of July 9, 2005 as observed by the MAGIC telescope, and derive the relevant parameters for the pre-flare quasi-steady-state and the states during the flare. We demonstrate that a hard lag flare can be obtained with parameters well inside the range expected for this source. In particular, a high value of Doppler factor appears necessary.
Dieckmann ME, Meli A, Shukla PK, Drury LOC, Mastichiadis A. Two-dimensional PIC simulations of ion beam instabilities in Supernova-driven plasma flows. [Internet]. 2008;50:065020. WebsiteAbstract
Supernova remnant (SNR) blast shells can reach the flow speed $v_s = 0.1 c$ and shocks form at its front. Instabilities driven by shock-reflected ion beams heat the plasma in the foreshock, which may inject particles into diffusive acceleration. The ion beams can have the speed $v_b \approx v_s$. For $v_b \ll v_s$ the Buneman or upper-hybrid instabilities dominate, while for $v_b \gg v_s$ the filamentation and mixed modes grow faster. Here the relevant waves for $v_b \approx v_s$ are examined and how they interact nonlinearly with the particles. The collision of two plasma clouds at the speed $v_s$ is modelled with particle-in-cell (PIC) simulations, which convect with them magnetic fields oriented perpendicular to their flow velocity vector. One simulation models equally dense clouds and the other one uses a density ratio of 2. Both simulations show upper-hybrid waves that are planar over large spatial intervals and that accelerate electrons to $\sim$ 10 keV. The symmetric collision yields only short oscillatory wave pulses, while the asymmetric collision also produces large-scale electric fields, probably through a magnetic pressure gradient. The large-scale fields destroy the electron phase space holes and they accelerate the ions, which facilitates the formation of a precursor shock.
Meli A, Mastichiadis A. Particle shock acceleration in astrophysics. [Internet]. 2008;588:193 - 196. WebsiteAbstract
The Fermi (diffusive) particle acceleration in astrophysical shocks is reviewed and evaluated. We discuss their properties and we present Monte Carlo simulations studying the shocks’ efficiency in accelerating particles (i.e. protons) up to very high energies with an application to astrophysical regions such as Supernovae, Active Galactic Nuclei hot spots and Gamma Ray Bursts. We find that the efficiency of the acceleration mechanism at shocks, varies in regard to the inclination of the magnetic field and the shock normal (e.g. sub-luminal shocks, super-luminal shocks), with consequences to the contribution of the very high energy particles to the observed cosmic ray spectrum.
Mastichiadis A, Kazanas D. The Supercritical Pile Model for GRBs:. the Prompt to Early Afterglow Stage. [Internet]. 2008;17:1641 - 1650. WebsiteAbstract
The "supercritical pile" is a very economical GRB model that provides for the efficient conversion of the energy stored in the protons of a relativistic blast wave (RBW) into radiation and at the same time produces — in the prompt GRB phase, even in the absence of any particle acceleration — a spectral peak at an energy ~ 1 MeV. We extend this model to include also the evolution of the RBW Lorentz factor Γ and thus follow the spectral and temporal features of this model into the GRB early afterglow stage. One of the novel features of the present treatment is the inclusion of the feedback of the GRB produced radiation on the evolution of Γ with radius. This way one can obtain afterglow light curves with steep decays followed by a relatively flatter flux stage, as observed in a large number of bursts.
2007
Mastichiadis A, Kazanas D. The supercritical pile model for GRBs. [Internet]. 2007;40:1250 - 1254. WebsiteAbstract
We present the basic notions of the "Supercritical Pile" model of Gamma Ray Bursts (GRBs). This model is motivated by the need for a process that provides the dissipation necessary in GRBs and presents a well defined scheme for converting the energy stored in the relativistic protons of the Relativistic Blast Waves (RBW) associated with GRB into radiation; at the same time it leads to spectra which exhibit a peak in the burst νFν distribution at an energy Ep ≃ 1 MeV in the observer's frame, in agreement with observation and largely independent of the Lorentz factor Γ of the associated relativistic outflow.
Moraitis K, Mastichiadis A. A two-zone model for the emission from RX J1713.7-3946. [Internet]. 2007;462:173 - 177. WebsiteAbstract
Aims:We study the acceleration and radiation of charged particles in the shock waves of supernova remnants using a recent version of the "box model". According to this, particles are accelerated in an energy-dependent region around the shock by the first order Fermi mechanism and lose energy through radiation. Methods: The particle distribution function is obtained from a spatially averaged kinetic equation that treats the energy losses self-consistently. There exists also a second population that consists of those particles that escape behind the shock where they also radiate. The energy distribution of this population is calculated in a similar manner. Results: The application of the model to the supernova remnant RX J1713.7-3946, which was recently confirmed as a TeV source by HESS, shows that the X-ray emission can be attributed to electron synchrotron radiation while in γ-rays there are contributions from both electrons and protons, with protons playing the dominant role. Additionally, there are strong indications that particles diffuse in turbulence that has a Kolmogorov spectrum.
2006
Magkanari M, Sapountzis K, Mastichiadis A, Vlahakis N. Radiation from internal shocks in magnetized supercritical flows. [Internet]. 2006;121. Website
Katarzyński K, Ghisellini G, Mastichiadis A, Tavecchio F, Maraschi L. Stochastic particle acceleration and synchrotron self-Compton radiation in TeV blazars. [Internet]. 2006;453:47 - 56. WebsiteAbstract
Aims.We analyse the influence of the stochastic particle acceleration for the evolution of the electron spectrum. We assume that all investigated spectra are generated inside a spherical, homogeneous source and also analyse the synchrotron and inverse Compton emission generated by such an object. Methods: .The stochastic acceleration is treated as the diffusion of the particle momentum and is described by the momentum-diffusion equation. We investigate the stationary and time dependent solutions of the equation for several different evolutionary scenarios. The scenarios are divided into two general classes. First, we analyse a few cases without injection or escape of the particles during the evolution. Then we investigate the scenarios where we assume continuous injection and simultaneous escape of the particles. Results: .In the case of no injection and escape the acceleration process, competing with the radiative cooling, only modifies the initial particle spectrum. The competition leads to a thermal or quasi-thermal distribution of the particle energy. In the case of the injection and simultaneous escape the resulting spectra depend mostly on the energy distribution of the injected particles. In the simplest case, where the particles are injected at the lowest possible energies, the competition between the acceleration and the escape forms a power-law energy distribution. We apply our modeling to the high energy activity of the blazar Mrk 501 observed in April 1997. Calculating the evolution of the electron spectrum self-consistently we can reproduce the observed spectra well with a number of free parameters that is comparable to or less than in the "classic stationary" one-zone synchrotron self-Compton scenario.
Mastichiadis A, Kazanas D. The Supercritical Pile Model for Gamma-Ray Bursts: Spectro-Temporal Properties. [Internet]. 2006;645:416 - 430. WebsiteAbstract
We present the spectral and temporal radiative signatures expected within the supercritical pile'' model of gamma-ray bursts (GRBs). This model is motivated by the need for a process that provides the dissipation necessary in GRBs and presents a well-defined scheme for converting the energy stored in the relativistic protons of the relativistic blast waves (RBWs) associated with GRBs into radiation; at the same time, it leads to spectra that exhibit a peak in the burst νFν distribution at an energy Epeak~=1 MeV in the observer's frame, in agreement with observations and largely independent of the Lorentz factor Γ of the associated relativistic outflow. Furthermore, this scheme does not require (but does not preclude) acceleration of particles at the shock other than that provided by the isotropization of the flow bulk kinetic energy in the RBW frame. In the present paper we model in detail the evolution of protons, electrons, and photons from a RBW to produce detailed spectra of the prompt GRB phase as a function of time from across a very broad range in frequency, spanning roughly 4logΓ decades. The model spectra are in general agreement with observations and provide a means for delineating the model parameters through direct comparison with trends observed in GRB properties.
2005
Katarzyński K, Ghisellini G, Tavecchio F, Maraschi L, Fossati G, Mastichiadis A. Correlation between the TeV and X-ray emission in high-energy peaked BL Lac objects. [Internet]. 2005;433:479 - 496. WebsiteAbstract
We discuss the correlation between the evolution of the TeV emission and X-ray radiation observed in high-energy peaked BL Lac objects. We describe such a correlation by a simple power law F_TeV(t) propto F^x_X-ray(t). In the first part of this work we present correlations obtained for the activity of Mrk 501 observed in 1997 April and for the activity of Mrk 421 observed in 2000 February. Our results obtained for Mrk 501 show that the index of the correlation (x) may strongly depend on the width and position of the spectral bands used for the comparison. The result of the correlation which we have obtained for Mrk 421 is not informative. However, we discuss results of similar correlation obtained for this source by other authors. They report an almost quadratic (x ∼ 2) correlations observed between the evolution of the TeV and X-ray emission. In the second part of this paper we present a phenomenological model which describes the evolution of the synchrotron and inverse Compton emission of a simple spherical homogeneous source. Neglecting the radiative cooling of the particles we derive analytical expressions that describe the evolution. Then we use a numerical code to investigate the impact of radiative cooling on the evolution. We show that different forms of correlations can be obtained depending on the assumed evolution scenario and the spectral bands used for the calculation. However, the quadratic correlation observed during the decay phase of the flare observed in Mrk 421 on 2001 March 19 appears problematic for this basic modeling. The quadratic correlation can be explained only for specific choices of the spectral bands used for the calculation. Therefore, looking for more robust solutions, we investigate the evolution of the emission generated by a cylindrical source. However this model does not provide robust solutions for the problem of a quadratic correlation. In principle the problem could be solved by the TeV emission generated by the self Compton scattering in the Thomson limit. However, we show that such a process requires unacceptably large values of the Doppler factor. Finally we briefly discuss the possible influence of the light travel time effect on our results.
Mastichiadis A, Protheroe RJ, Kirk JG. Spectral and temporal signatures of ultrarelativistic protons in compact sources. I. Effects of Bethe-Heitler pair production. [Internet]. 2005;433:765 - 776. WebsiteAbstract
We present calculations of the spectral and temporal radiative signatures expected from ultrarelativistic protons in compact sources. The coupling between the protons and the leptonic component is assumed to occur via Bethe-Heitler pair production. This process is treated by modeling the results of Monte-Carlo simulations and incorporating them in a time-dependent kinetic equation, that we subsequently solve numerically. Thus, the present work is, in many respects, an extension of the leptonic “one-zone” models to include hadrons. Several examples of astrophysical importance are presented, such as the signature resulting from the cooling of relativistic protons on an external black-body field and that of their cooling in the presence of radiation from injected electrons. We also investigate and refine the threshold conditions for the “Pair Production/Synchrotron” feedback loop which operates when relativistic protons cool efficiently on the synchrotron radiation of the internally produced Bethe-Heitler pairs. We demonstrate that an additional component of injected electrons lowers the threshold for this instability.
2004
Kazanas D, Georganopoulos M, Mastichiadis A. The supercritical pile model of GRB: tapping the proton energy and getting the νFν peak at ∼1 MeV. [Internet]. 2004;48:493 - 495. WebsiteAbstract
We propose a process by which the kinetic energy of the protons, that carry most of the energy of GRB relativistic blast waves (RBW), is converted explosively into relativistic electrons of the same Lorentz factor, which then produce the observed prompt γ-ray emission of the burst. This conversion is the result of the combined effects of the reflection of photons produced within the flow by upstream located matter, their re-interception and conversion into e +e --pairs on the RBW by the pγ→ pe +e - reaction. This process depends on kinematic and dynamic thresholds; the former depends on the RBW Lorentz factor Γ while the second on the column density of the post shock matter to the pγ→e +e - reaction; this latter condition is in effect that of the criticality of a nuclear pile, hence the erminology. It is shown that, when operating near threshold, the resulting GRB spectrum produces its peak luminosity at energy (in the lab frame) E≃ mec2, thereby providing an answer to this outstanding question of GRBs.
Georganopoulos M, Kirk JG, Mastichiadis A. . [Internet]. 2004;604:479 - 479. WebsiteAbstract
Equation (5) in § 2 should readf(x)=[2xlnx+x+1-2x2+(4∊0γx)22(1+4∊0γx)(1-x)]P(1/4γ2,1,x),x=∊4∊0γ2(1-∊/γ),(5)as given by F. C. Jones (ApJ, 561, 111 [2001]). In our original paper, the factor (1-x) in the final term inside the bracket was missing. The correct form was used in all computations.
2003
Konopelko A, Mastichiadis A, Kirk J, de Jager OC, Stecker FW. Modeling the TeV Gamma-Ray Spectra of Two Low-Redshift Active Galactic Nuclei: Markarian 501 and Markarian 421. [Internet]. 2003;597:851 - 859. WebsiteAbstract
We discuss the results of modeling the TeV γ-ray spectra of two active galactic nuclei, Mrk 501 and Mrk 421, that have almost the same redshifts: z=0.031 and 0.034, respectively. The effect of intergalactic γ-ray absorption is treated as an uncertainty in the measurement of the intrinsic spectrum. Although the objects differ, we obtain satisfactory fits for both of them in a synchrotron self-Compton scenario. Compared to previous models, our fits are characterized by higher values of the Doppler factor (δ>=50) and an electron injection spectrum extending to higher energies (γmax>=1.5×105). In the case of Mrk 421, the observed difference in spectral slope in X-rays and TeV γ-rays between the high and low states can be explained as a variation of a single parameter-the maximum energy γmaxmc2 at which electrons are injected.
Rokaki E, Lawrence A, Economou F, Mastichiadis A. Is there a disc in the superluminal quasars?. [Internet]. 2003;340:1298 - 1308. WebsiteAbstract
We look for the expected signature of an accretion disc by examining the properties of the Hα emission line versus viewing angle in a sample of 22 superluminal (SL) quasars. The Doppler factor δ, jet velocity γ and viewing angle θ towards the jet are derived from published radio and X-ray data. Most of the Hα spectra (14) have been observed at the United Kingdom Infrared Telescope (UKIRT) and are reported here. About a quarter of the SL objects have weak or absent Hα emission lines, with small equivalent widths (EW). These have high optical polarization, radio core dominance and Doppler factor, and most of them have high apparent SL velocity and low viewing angles. Therefore these weak-EW objects almost certainly have relativistically beamed optical continua. The strong-EW objects also show a clear beaming effect, but a much weaker one, with line EW varying by only a factor of 3 while radio core dominance varies by a factor of several hundred. The correlation of EW with θ is quantitatively in good agreement with the prediction of a flat accretion disc with limb darkening. The weak- and strong-EW sources also show an anticorrelation of line velocity width with the various beaming indicators. Again, the correlation with the derived viewing angle θ shows a quantitative agreement with the effect expected for an axisymmetric structure with velocity dominated by rotation. The line emission cannot come from the surface of the disc, or the line beaming would cancel the continuum beaming. However, it could come from an axisymmetric system of clouds corotating with the accretion disc.
2002
Kazanas D, Georganopoulos M, Mastichiadis A. The Supercritical Pile'' Model for Gamma-Ray Bursts: Getting the νFν Peak at 1 MeV. [Internet]. 2002;578:L15 - L18. WebsiteAbstract
We propose that the internal energy of the gamma-ray burst (GRB) blast waves, thought to be stored in the form of relativistic protons comoving with the blast wave, is converted explosively (i.e., on light crossing timescales) into relativistic electrons of the same Lorentz factor, which are responsible for the production of observed prompt γ-ray emission of the burst. This conversion is the result of the combined effects of the reflection of photons produced within the flow by upstream located matter, their reinterception by the blast wave, and their eventual conversion into e+e- pairs in interactions with the relativistic protons of the blast wave (via the pγ-->pe+e- reaction). This entire procedure is contingent on two conditions on the relativistic protons: a kinematic one imposed by the threshold of the pγ-->pe+e- reaction and a dynamic one related to the column density of the postshock matter to the same process. This latter condition is in essence identical to that of the criticality of a nuclear pile, hence the terminology. It is argued that the properties of relativistic blast waves operating under these conditions are consistent with GRB phenomenology, including the recently found correlation between quiescence periods and subsequent flare fluence. Furthermore, it is shown that, when operating near threshold, the resulting GRB spectrum produces its peak luminosity at an energy (in the lab frame) E~=mec2, thereby providing an answer to this outstanding question of GRBs.
Akylas A, Georgantopoulos I, Griffiths RG, Papadakis IE, Mastichiadis A, Warwick RS, Nandra K, Smith DA. Monitoring RXTE observations of Markarian 348: the origin of the column density variations. [Internet]. 2002;332:L23 - L27. WebsiteAbstract
We analyse 37 RXTE observations of the type 2 Seyfert galaxy Mrk 348 obtained over a period of 14 months. We confirm the spectral variability previously reported by Smith, Georgantopoulos & Warwick, in the sense that the column density decreases by a factor of ~3 as the count rate increases. Column density variations could possibly originate either from the random drift of clouds within the absorption screen, or from photoionization processes. Our modelling of the observed variations implies that the first scenario is more likely. These clouds should lie within a distance of >2 light-years from the source, having a diameter of a few light-days and a density of >107 cm-3 , hence probably residing outside the broad-line region.
Mastichiadis A, Kirk JG. Models of Variability in Blazar Jets. [Internet]. 2002;19:138 - 142. WebsiteAbstract
During the last decade multiwavelength observations of blazars have revealed many interesting patterns in their emission across the EM spectrum. In the present article we will review the time-dependent one-zone models and the models which advocate an acceleration and a radiation zone, and we will make some comparisons between them, especially in light of recent observations of the so-called TeV blazars.
Mastichiadis A. Radiative Processes in Relativistic Outflows. In: Vol. 589. ; 2002. pp. 1. WebsiteAbstract
1. Introduction 2. Superlumminal Motion 3. Doppler Boosting 4. Photon-Photon Pair Binding 5. Synchrotron Radiation 6. Inverse Compton Scattering 6.1. Thomson Scattering 6.2. Klein-Nishina Limit 7. Synchro Self-Compton Radiation 8. Continuity Equation 9. Coda References
2001
Georganopoulos M, Kirk JG, Mastichiadis A. The Beaming Pattern and Spectrum of Radiation from Inverse Compton Scattering in Blazars. [Internet]. 2001;561:111 - 117. WebsiteAbstract
By including Klein-Nishina effects, we generalize previous calculations of the beaming pattern of photons produced by inverse Compton scattering. For an isotropic distribution of soft photons upscattered by nonthermal electrons with a power-law density distribution n(γ)~γ-p, embedded in a plasma moving with relativistic bulk speed, we show that the observed radiation intensity is proportional to D3+p, where D is the Doppler boosting factor. This agrees with previous computations performed in the Thomson limit, where the observed spectral index is α=(p-1)/2 and the beaming pattern is D4+2α. Independent of D, Klein-Nishina effects limit the location of the peak energy ∊peakmec2 of the observed spectral energy distribution such that ∊peak<~1/∊0, where ∊0 is the energy of the seed photons in units of mec2. Assuming that the seed photons originate in the broad-line region, we demonstrate that the GeV emission of blazars is significantly modified by Klein-Nishina effects, the spectrum being softer than that calculated in the Thomson limit. We further show that the change in the spectral index of the inverse Compton emission across ∊peak can exceed the value of 0.5 predicted by computations performed in the Thomson limit. The model spectra agree with OSSE and COMPTEL limits on this break without invoking the effects of differential absorption at the edge of a gamma-ray photosphere.
1999
Drury LO'C, Duffy P, Eichler D, Mastichiadis A. On box'' models of shock acceleration and electron synchrotron spectra. [Internet]. 1999;347:370 - 374. WebsiteAbstract
The recent detection of high energy gamma -rays coming from supernova remnants and active galactic nuclei has revived interest in the diffusive shock acceleration of electrons. In the present paper we examine the basis of the so-called box'' model for particle acceleration and present a more physical version of it. Using this we determine simple criteria for the conditions under which pile-ups'' can occur in shock accelerated electron spectra subject to synchrotron or inverse Compton losses (the latter in the Thompson limit). An extension to include nonlinear effects is proposed.
Konopelko AK, Kirk JG, Stecker FW, Mastichiadis A. Evidence for Intergalactic Absorption in the TEV Gamma-Ray Spectrum of Markarian 501. [Internet]. 1999;518:L13 - L15. WebsiteAbstract
The recent High-Energy Gamma-Ray Array (HEGRA) observations of the blazar Mrk 501 show strong curvature in the very high energy γ-ray spectrum. Applying the γ-ray opacity derived from an empirically based model of the intergalactic infrared background radiation field to these observations, we find that the intrinsic spectrum of this source is consistent with a power law: dNγ/dE~E-α, with α=2.00+/-0.03 over the range 500 GeV-20 TeV. Within current synchrotron self-Compton scenarios, the fact that the TeV spectral energy distribution of Mrk 501 does not vary with luminosity, combined with the correlated, spectrally variable emission in X-rays as observed by the BeppoSAX and Rossi X-Ray Timing Explorer instruments, also independently implies that the intrinsic spectrum must be close to α=2. Thus, the observed curvature in the spectrum is most easily understood as resulting from intergalactic absorption.
Kazanas D, Mastichiadis A. The origin of TeV electrons in blazars. [Internet]. 1999;11:41 - 44. WebsiteAbstract
There is increasing observational evidence that relativistic particles of energies ∼1 TeV provide a significant pressure component of the plasma which powers at least some of the relativistic jets associated with AGN. Furthermore, observations of flares with duration ∼15 min at TeV energies indicate that the associated electrons are accelerated to the required energies on these or shorter time scales, which are comparable to the synchrotron loss time for the values of the magnetic fields thought present in these jets. As such, they push the potential acceleration mechanisms to their limits and prompt us to examine the conditions under which it may be possible for hadronic processes to provide the electrons of the requisite energies. Relativistic hadrons could presumably exist within the flow, having been accelerated efficiently near the compact object and then transported along with it, releasing their energy by an instability due to p- γ reactions once a well-defined threshold is reached.
Kazanas D, Mastichiadis A. Relativistic Electrons in Blazars: A Hadronic Origin?. [Internet]. 1999;518:L17 - L20. WebsiteAbstract
Recent observations of blazars have established that their γ-ray emission is associated, as a rule, with very fast variability (as short as ~15 minutes for the TeV photons of Mrk 421); as such, these observations push the theoretical models for the production of the required relativistic electrons to their limits. Herein we investigate the possibility that blobs'' loaded with relativistic protons could produce such an activity. We show that, if the proton number density in a blob exceeds a certain critical value, then reflection of its own synchrotron produced photons on some external mirror,'' such as a line-emitting cloud, can initiate a feedback process in which the protons can lose most of their energy content in a blob crossing time, resulting in a flare of the same duration. By performing a dimensional analysis, we find the necessary conditions for such an instability to occur, and we show that the conditions required are consistent with those usually assumed to prevail within the relativistic jets of this class of active galactic nuclei.
Kirk JG, Mastichiadis A. Variability patterns of synchrotron and inverse Compton emission in blazars. [Internet]. 1999;11:45 - 48. WebsiteAbstract
The acceleration of electrons at a shock front can produce characteristic patterns in the variation of the spectral index of the synchrotron emission as a function of flux. Using a simple model of the acceleration process, we present a discussion of these patterns and show how they compare with the variations in the emission of the same electrons via inverse Compton scattering of isotropically distributed target photons from an external source. The "soft lag" behaviour is observed in synchrotron emission, and should also be present in the inverse Compton flux. Shock models can also show "hard lag" behaviour of the synchrotron emission, but this is more difficult to achieve in the inverse Compton emission, because of Klein-Nishina effects. In some cases, the time scales of rise and fall of both the synchrotron and inverse Compton fluxes can depend on the acceleration mechanism.
Kazanas D, Mastichiadis A. The Origin of Tev Electrons in Blazars. [Internet]. 1999;39:29. Website
1998
Kirk JG, Rieger FM, Mastichiadis A. Particle acceleration and synchrotron emission in blazar jets. [Internet]. 1998;333:452 - 458. WebsiteAbstract
We model the acceleration of electrons at a shock front in a relativistic blazar jet and compute the radiation they emit in a post-shock region which contains a homogeneous magnetic field. The full space, time and momentum dependence of the electron distribution is used in this calculation. It is shown that the  homogeneous'\ synchrotron model is recovered, provided the downstream speed of the plasma away from the shock front is nonrelativistic, and provided that the light travel times across the face of the shock front is unimportant. By varying the rate at which particles are picked up by the acceleration process, we calculate the time-dependence of the spectra. Since the magnetic field strength is assumed constant within the emission region, each frequency band can be identified with electrons of a particular energy. We find that for a band in which the electrons are accelerated rapidly compared to the rate at which they cool, the spectra typically harden during phases of rising flux, and soften during phases of falling flux, as has been observed in the objects PKS 2155-304 and Mkn 421. However, in a frequency band in which the timescales are comparable, the reverse behaviour is to be expected. We discuss the extent to which observations of both the stationary spectrum and the spectral variability of the synchrotron component of blazar emission can be used to constrain the model.
1997
Titarchuk L, Mastichiadis A, Kylafis ND. X-Ray Spectral Formation in a Converging Fluid Flow: Spherical Accretion into Black Holes. [Internet]. 1997;487:834 - 846. WebsiteAbstract
We study Compton upscattering of low-frequency photons in a converging flow of thermal plasma. The photons escape diffusively, and electron scattering is the dominant source of opacity. We solve the equation of radiative transfer in the case of spherical, steady state accretion into black holes numerically and approximately analytically. Unlike previous work on this subject, we consider the inner boundary at a finite radius, and this has a significant effect on the emergent spectrum. It is shown that the bulk motion of the converging flow is more efficient in upscattering photons than thermal Comptonization, provided that the electron temperature in the flow is of order a few keV or less. In this case, the spectrum observed at infinity consists of a soft component coming from input photons that escaped after a few scatterings without any significant energy change and of a power law that extends to high energies and is made of those photons that underwent significant upscattering. The luminosity of the power law is relatively small compared to that of the soft component. The more reflective the inner boundary is, the flatter the power-law spectrum becomes. The spectral energy power-law index for black hole accretion is always higher than 1, and it is approximately 1.5 for high accretion rates. This result tempts us to say that bulk motion Comptonization might be the mechanism behind the power-law spectra seen in black hole X-ray sources.
de Jager OC, Mastichiadis A. A Relativistic Bremsstrahlung/Inverse Compton Origin for 2EG J1857+0118 Associated with Supernova Remnant W44. [Internet]. 1997;482:874 - 880. WebsiteAbstract
We show that relativistic bremsstrahlung and inverse Compton scattering of radio-emitting electrons can easily account for the observed γ-ray spectrum of 2EG J1857+0118 if the field strength in the shell is below ~30 μG. This source is located at the eastern border of the composite SNR W44, where the expanding radio shell is interacting with a dense molecular cloud. The nondetection of this remnant above 250 GeV implies a cutoff or steepening in the electron spectrum above ~100 GeV. The E-1.66 spectrum of this radio/γ-ray-emitting electron component is too flat to have its origin in standard first-order Fermi acceleration, but electron injection into the shell by the pulsar PSR B1853+01 over the 2 × 104 yr lifetime may explain why the Crab-like radio spectrum (Sν ~ ν-0.33) is about the hardest of all shell-type remnants. The injected energy would be sufficient to account for the required energy of 6 × 1049 ergs if the initial spin-down power of PSR B1853+01 was about 10 times larger than the present spin-down power of the Crab pulsar. A steeper Fermi electron component may be present, but the observational data are not constraining enough to provide a meaningful limit on the presence of an additional ~E-2 shell-type electron component. The predicted γ-ray contribution from high-energy proton-gas interactions is about 20% of the observed EGRET flux above 100 MeV, which confirms our conclusion that the γ-ray emission from W44 is dominated by a leptonic component.
Mastichiadis A, Kirk JG. Variability in the synchrotron self-Compton model of blazar emission. [Internet]. 1997;320:19 - 25. WebsiteAbstract
We present a model of the spectra of gamma-ray emitting blazars in which a single homogeneous emission region both emits synchrotron photons directly and scatters them to high (gamma-ray) energy before emission (a synchrotron self-Compton' or SSC model). In contrast to previous work, we follow the full time dependent evolution of the electron and photon spectra, assuming a power-law form of the electron injection and examine the predictions of the model with regard to variability of the source. We apply these computations to the object Mkn 421, which displayed rapid variability in its X-ray and TeV emission during a multiwavelength campaign in 1994. This observation strongly implies that the same population of electrons produces the radiation in both energy bands. By fitting first the observed quiescent spectrum over all 18 orders of magnitude in frequency, we show that the time dependence of the keV/TeV flare could have been the result of a sudden increase in the maximum energy of the injected electrons. We show also that different types of flare may occur in this object and others, and that the energy band most sensitive to the properties of the acceleration mechanism is the X-ray band.
1996
Bednarek W, Kirk JG, Mastichiadis A. Production of gamma-rays by inverse Compton scattering in jets. [Internet]. 1996;120:571 - 574. WebsiteAbstract
We discuss a model for the γ-ray production in blazars in which electrons are accelerated rectilinearly in localised regions of the jet, scattering soft radiation from the accretion disk. In our model the jet divides naturally into two zones. In the radiation dominated zone' (close to the disk), the acceleration of electrons is balanced by inverse Compton losses in the Thomson regime and energy is efficiently transferred into the γ-rays. The γ-ray spectral slope is determined by the electric field profile along the jet. In the particle dominated zone' (further from the disk) the electron losses are too low to balance acceleration and the electrons are injected into the jet with energies corresponding to the full potential drop in the acceleration region. We suggest that these electrons are then isotropised by the random component of the magnetic field of the jet and cool mainly by synchrotron losses. In the framework of our model we predict further that Galactic black hole candidates might be sources of γ-radiation; in this case, however, we do not expect emission above 10GeV.
Titarchuk L, Mastichiadis A, Kylafis ND. Spherical accretion onto neutron stars and black holes. [Internet]. 1996;120:171 - 174. WebsiteAbstract
Spectral formation in steady state, spherical accretion onto neutron stars and black holes is examined by solving numerically and analytically the equation of radiative transfer. The photons escape diffusively and their energy gains come from their scattering off thermal electrons in the converging flow of the accreting gas. We show that the bulk motion of the flow is more efficient in upscattering photons than thermal Comptonization in the range of non-relativistic electron temperatures. The spectrum observed at infinity is a power law with an exponential turnover at energies of order of the electron rest mass. Especially in the case of accretion into a black hole, the spectral energy power-law index is distributed around 1.5. Because bulk motion near the horizon (1-5 Schwarzschild radii) is most likely a necessary characteristic of accretion into a black hole, we claim that observations of an extended power law up to about m_e_c^2^, formed as a result of bulk motion Comptonization, is a real observational evidence for the existence of an underlying black hole.
Mastichiadis A, de Jager OC. TeV emission from SN 1006. [Internet]. 1996;311:L5 - L8. WebsiteAbstract
Supernova 1006 is the first shell type supernova remnant to show evidence of particle acceleration to TeV energies. In the present paper we examine this possibility by modeling the observed X-ray non-thermal emission in terms of synchrotron radiation from Fermi accelerated electrons. The predicted synchrotron spectrum fits the radio and non-thermal component of the observed soft X-ray to hard X-ray emission quite well. These particles can produce TeV gamma rays by inverse Compton scattering on the microwave radiation and other ambient fields, and the derived electron distribution is also used to calculate the expected inverse Compton flux. We find that if the remnant is characterised by a magnetic field strength lower than ~7yG, then the TeV flux can be higher than that of the Crab Nebula. About 75% of the TeV emission from SN 1006 is expected to be concentrated in the synchrotron bright NE and SW rims (the "hard aegis") of the remnant, which would allow a sensitive search if the Atmospheric Imaging Cherenkov Technique is used.
Bednarek W, Kirk JG, Mastichiadis A. On the production of very high energy beamed gamma-rays in blazars. [Internet]. 1996;307:L17. WebsiteAbstract
The variable flux of TeV gamma-rays detected from Mkn 421 and Mkn 501 requires the presence of high energy electrons, which could in principle produce large numbers of electron/positron pairs, leading to an electromagnetic cascade. We point out that this scenario can be avoided if electrons are accelerated to high energy rectilinearly, rather than being injected isotropically into a blob, as in most of the models of the GeV gamma-ray emission. By balancing linear acceleration by an electric field against inverse Compton losses in the radiation field of the accretion disk we calculate the emitted spectra and find the conditions which must be fulfilled in order to exclude the development of electromagnetic cascades during acceleration. Assuming these to be fulfilled, we show that the maximum possible photon energy is approximately 10M_8_^2/5^TeV, where M_8_ is the mass of the central black hole in units of 10^8^Msun_. In addition we compute the optical depth to absorption of TeV photons on a possible isotropic scattered component and on the observed nonthermal radiation (in the case of Mkn 421) and find that TeV photons can escape provided the nonthermal X-rays originate in a jet moving with a Lorentz factor γ_b_>8.
Mastichiadis A. The Hadronic Model of Active Galactic Nuclei. [Internet]. 1996;75:317 - 329. WebsiteAbstract
We review the hadronic model for Active Galactic Nuclei (AGN). This model, which can be applied to all AGN, advocates the acceleration of protons to ultrarelativistic energies by shock fronts which are formed a few Schwarzschild radii away from the central black hole. The necessary consequences of this hypothesis are discussed. These include the formation of electromagnetic cascades which are initiated by the injection of secondary electrons and photons inside the source, as well as the production and escape of neutrons and neutrinos. As a result of the neutron escape we emphasize that AGN can be sources of TeV radiation.
Mastichiadis A. On the high energy non-thermal emission from shell-type supernova remnants. [Internet]. 1996;305:L53. WebsiteAbstract
Shock waves associated with shell type supernova remnants are considered to be possible sites of cosmic ray acceleration. Since shocks are capable of accelerating electrons in addition to protons one anticipates both species to contribute to the high energy radiation expected from these objects. Adopting a simple model for particle acceleration we calculate in a self-consistent manner the time-dependent synchrotron and inverse Compton radiation of high energy electrons assumed either to be accelerated directly by the shock wave or to be injected at high energies as secondaries from the hadronic collisions of relativistic protons with the circumstellar material. We deduce that for standard supernova parameters the TeV flux produced from neutral pion decay is about the same order as the flux expected from directly accelerated electrons.
1995
Mastichiadis A, Kirk JG. Self-consistent particle acceleration in active galactic nuclei. [Internet]. 1995;295:613. WebsiteAbstract
Adopting the hypothesis that the nonthermal emission of Active Galactic Nuclei (AGN) is primarily due to the acceleration of protons, we construct a simple model in which the interplay of acceleration and losses can be studied together with the formation of the emitted spectrum. The acceleration process is assumed to be of the first order Fermi type, and the proton distribution as well as the injected electrons and photons in the central region of the AGN are described by spatially averaged kinetic equations. The various relevant processes which dominate the three species are incorporated into the equations. The technique used to solve these is presented and several tests of the numerical implementation are presented. We also present results of a sample time-dependent AGN model in which photons appear suddenly as a result of a feedback instability and the system evolves to a steady state, in which the acceleration process is saturated self-consistently by the photons it produces. This example combines an X-ray power law index of about -1.7, together with a break at an energy between 50 and 500keV.
1994
Mastichiadis A, Protheroe RJ, Szabo AP. The Effect of Triplet Production on Pair / Compton Cascades in Thermal Radiation. [Internet]. 1994;266:910. WebsiteAbstract
We calculate the spectrum of photons resulting from electromagnetic cascades through thermal radiation, and examine the consequences of including triplet production in these cascades. We assume that the cascade is one-dimensional, and we find that this approximation is justified in the present work for thermal radiation with temperature less than 10-3 mc2. Results are obtained for both monoenergetic and power-law primary spectra, and for a variety of path lengths. We find that triplet production is particularly important in electron-photon cascades through thermal radiation when the primary energy exceeds 105m2c4/kT for propagation over small path lengths. The importance of triplet production decreases as the path length increases, and it has no effect on saturated cascades.
Titarchuk L, Mastichiadis A. Hard X-Rays from NGC 4151: A Thermal Origin?. [Internet]. 1994;433:L33. WebsiteAbstract
We present a model for explaining the recent combined X-ray and low- energy gamma-ray observations of the Seyfert galaxy NGC 4151. According to this model, soft photons become Comptonized in a hot spot producing simultaneously the low-energy power law as observed by Ginga and the high-energy cutoff observed by OSSE. Implementing recently developed theoretical calculations toward a generalized theory of Comptonization, we were able to find fits to the observations using only two parameters which characterize the physical quantities of the emission region: the plasma cloud optical depth and its temperature. We find that there is no need for additional nonthermal, reflection, or higher temperature thermal components to fit the aforementioned OSSE and Ginga observations. We derive in addition the size of the photon region and the temperature of the upscattered soft photons. We should emphasize, also, that any attempt at fitting only the high-energy parts of the spectrum (photon energies > 60 keV) by the Sunyaev & Titarchuk (1980) nonrelativistic Comptonization model leads to an underestimate of the Comptonization parameter γ (or, equivalently, to an overestimation of the X-ray power-law spectral slope) and leads, as a result, to incorrect proportions between the low-energy and high-energy parts of the spectrum.
Johnson PA, Mastichiadis A, Protheroe RJ, Stanev TS, Szabo AP. The contribution of active Galactic nuclei to the diffuse gamma-ray background. [Internet]. 1994;20:979 - 990. WebsiteAbstract
We consider the emission of high energy to very high energy $\gamma$-rays in radio-quiet active galactic nuclei (AGN) or the central regions of radio-loud AGN. We use our results to estimate the $\gamma$-ray flux from the central regions of nearby AGN, and then to calculate the contribution to the diffuse $\gamma$-ray flux from unresolved AGN.
Mastichiadis A, Ozernoy LM. X-Ray and Gamma-Ray Emission of Sagittarius A * as a Wind-accreting Black Hole. [Internet]. 1994;426:599. WebsiteAbstract
If, as many believe, Sgr A* is a massive black hole at the Galactic center, one should expect it to be a source of X-ray and gamma-ray activity, behaving basically as a scaled-down active galactic nucleus. An unavoidable source of accretion is the wind from IRS 16, a nearby group of hot, massive stars. Since the density and velocity of the accreting matter are known from observations, the accretion rate is basically a function of the putative black hole mass, Mh, only; this value represents a reliable lower limit to a real rate, given the other possible sources of accreting matter. Based on this and on the theories about shock acceleration in active galactic nuclei, we have estimated the expected production of relativistic particles and their hard radiation. These values turn out to be a function of Mh as well. Comparing our results with available X-ray and gamma-ray observations which show Sgr A* to have a relatively low activity level, we conclude tentatively that the putative black hole in the Galactic center cannot have a mass greater than approximately 6 x 103 solar mass. This conclusion is consistent with the upper limits to the black hole mass found by different methods earlier, although much more work is needed to make calculations of shock acceleration around black holes more reliable.
1992
Protheroe RJ, Mastichiadis A, Dermer CD. Pair-Compton cascading in a spatially varying anisotropic radiation field. [Internet]. 1992;1:113 - 127. WebsiteAbstract
When energetic electrons Compton scatter soft photons in the Klein-Nishina regime, the produced gamma rays are subject to photon-photon pair attenuation, and a pair-Compton cascade results. We examine the formation of such cascades in the spatially varying photon fields encountered by high-energy electrons and photons emitted from luminous compact objects. Constraints on the geometry of the soft photon source and the location of the gamma-ray production site are deduced from observations of 100 MeV and TeV gamma rays emitted by galactic compact objects and active galactic nuclei.
Kirk JG, Mastichiadis A. X-ray flares from runaway pair production in active galactic nuclei. [Internet]. 1992;360:135 - 137. WebsiteAbstract
ACTIVE galactic nuclei (AGNs) exhibit high luminosity with rapid variability, especially in X-ray emission, for which the luminosity can be greater than that of a normal galaxy, and the variability timescale implies an emitting region in some cases smaller than one light hour1. The hard X-ray spectrum of AGNs is nonthermal, probably arising from an electron-positron pair cascade, with some emission reflected off relatively cold matter2,3. Energy can be pumped into a cascade by any process that produces relativistic pairs, but because electrons and positrons are hard to accelerate efficiently, there has been interest in models in which protons are accelerated4,5, and create relativistic electrons on interaction with a local radiation field6-8. Here we show that a sufficient column density of protons can lead to runaway pair production: photons generated by the relativistic pairs are the targets for the protons to produce more pairs. This process can produce X-ray flares with the observed characteristics, and our model predicts the maximum ratio of luminosity to source size ('compactness') as well as their spectrum in the early phases. The same mechanism may also be able to create the knots of synchrotron-radiating pair plasma seen in sources such as 3C273.
Livio M, Mastichiadis A, Oegelman H, Truran JW. On the Role of Radioactive Decays in Powering Gamma Rays and X-Rays from Novae. [Internet]. 1992;394:217. WebsiteAbstract
Existing models for X-ray emission from novae in outburst encounter several difficulties. The X-ray flux that may be expected to result from Compton degradation of gamma rays produced by radioactive decays is calculated. It is demonstrated that radioactive decays of Na-22 may play an interesting role in the production of hard X-rays, particularly in novae enriched in oxygen, neon, and magnesium.
Dermer CD, Schlickeiser R, Mastichiadis A. High-energy gamma radiation from extragalactic radio sources. [Internet]. 1992;256:L27 - L30. WebsiteAbstract
We propose that the important relationship between 3C 273 and 3C 279, the first two extragalactic sources detected at > 100 MeV energies, is their superluminal nature. In support of this conjecture, we propose a kinematic focusing mechanism, based on Compton scattering of accretion-disk photons by relativistic nonthermal electrons in the jet, that preferentially emits gamma rays in the superluminal direction.
Mastichiadis A, Kylafis ND. Compton Scattering in a Converging Fluid Flow: Spherical Near-critical Accretion onto Neutron Stars. [Internet]. 1992;384:136. WebsiteAbstract
Compton scattering of low-frequency photons in a converging flow of cold plasma is studied. The equation of radiative transfer in the case of spherical near-critical steady state accretion onto a neutron star is solved analytically. The inner boundary condition is that the neutron star surface is completely reflective, or that there is a magnetopause with an empty cavity inside it. The photons escape diffusively and electron scattering is the dominant source of opacity. The energy gain of the photon comes entirely from the bulk motion of the converging flow of the accreting gas. The spectrum observed at infinity is a power law at high frequencies with photon number spectral index essentially -1. This spectrum is significantly flatter than that found for accretion into black holes.
1991
Mastichiadis A. Relativistic electrons in photon fields - Effects of triplet pair production on inverse Compton gamma-ray spectra. [Internet]. 1991;253:235 - 244. WebsiteAbstract
Pair production of a photon in the Coulomb field of an electron (often referred to as triplet pair production) despite the fact that it is a third-order QED process has a cross-section that exceeds the Compton cross-section for energies of collision equal to or greater than 250 m(e)c-squared. The present paper reexamines the problem of ultrarelativistic electrons entering a photon field by including triplet pair production as both an energy loss and reinjection mechanism for electrons. The steady state electron distribution and photon spectrum are calculated for monoenergetic and power-law electron injections, and a comparison is made with the results obtained in the pure Compton case, i.e., by ignoring triplet pair production.
Harding AK, Mastichiadis A, Protheroe RJ, Szabo AP. Cosmic-Ray Transport and Gamma-Ray Emission in Supernova Shells. [Internet]. 1991;378:163. WebsiteAbstract
The mixing and transport of cosmic rays accelerated by a pulsar inside an expanding supernova remnant are examined, and the resulting high-energy gamma-ray emission from nuclear interactions of these accelerated particles in the shell is investigated. Rayleigh-Taylor instability at the interface between a pulsar wind cavity and the inner supernova envelope is assumed to be the mixing mechanism. The analysis is applied to the model of Gaisser, Harding, and Stanev (1987), where protons are accelerated at the reverse shock in the pulsar wind. The instability time-scale is estimated from the dynamics of the pulsar wind cavity, and model the injection, diffusion, and interaction of protons in the shell. The resulting gamma-ray flux is lower than previous estimates due to proton adiabatic losses in the expanding pulsar wind. The protons mix and diffuse only into the innermost regions of the envelope before interacting. Energy-dependent diffusion causes the higher energy gamma-ray light curves to decay faster than those at lower energy.
Mastichiadis A, Protheroe RJ, Stephens SA. Cosmic ray positron production by gamma ray interactions on starlight. [Internet]. 1991;9:115 - 117. WebsiteAbstract
The production of cosmic ray positrons by photon-photon pair production of high-energy gamma-rays on starlight photons is examined. Calculating the production rate as a function of positron energy and distance from the sun resulting from interactions with sunlight is the first step. The results are generalized to production on other types of star. The average production rate per unit volume averaged over the local region of the galaxy is calculated, and the contribution to the observed intensity from this process is estimated.
1990
Mastichiadis A, Protheroe RJ. VHE gamma-ray emission from active galactic nuclei. [Internet]. 1990;246:279. WebsiteAbstract
High-energy neutrons produced as a consequence of proton acceleration in active galactic nuclei (AGNs) can carry a substantial fraction of the initial luminosity outside of the central source. Part of this luminosity is expected to go into VHE and UHE γ-ray emission as a result of hadronic interactions of these relativistic particles with the accreting plasma. We find that the most efficient emitters will be AGNs with luminosities ~0.01 - 0-1 of the Eddington luminosity.
1989
Kirk JG, Mastichiadis A. Neutrons from active galactic nuclei. [Internet]. 1989;213:75 - 79. WebsiteAbstract
A recently proposed acceleration mechanism for ultra relativistic particles in the central engines of AGN's suggests that protons are the particles which are accelerated initially, and that these subsequently inject the electrons and positrons supposed responsible for the continuum emission. This model necessarily leads to the prediction of a strong flux of neutrons (with a luminosity at least as great as that in the photon continuum) from the central engine. We investigate some of the consequences of such a model. In particular, the neutron flux will, by means of spallation reactions, produce amounts of boron in the surrounding material which it may be possible to observe in the optical spectrum. In addition, the central engine will act as a source of high energy gamma-rays. Those which can propagate to the Earth (i.e., those with energy < 10^14^ eV) may be observed directly, whereas those of higher energy will be absorbed by radio photons from either the microwave background or the AGN itself. If this occurs in a typical interstellar magnetic field, the energy of the gamma-rays will be reradiated in the form of X-rays.
Mastichiadis A, Kylafis N, Ventura J. Supernova 1987A : envelope metallicity and the nature of the soft X-ray component. [Internet]. 1989;208:L11 - L14. WebsiteAbstract
The observed X-ray spectra reported by the GINGA and MIR-HEXE satellite experiments contain information on the metallicity of the Supernova's expanding envelope. Using a Monte Carlo code to simulate the Compton degradation of the ^56^Co -produced γ-rays, we calculate the emerging X-ray spectrum for various metallicities. The observed spectra are compatible with a low (less than solar) metal abundance, while an additional soft component, independent of the ^56^Co source is probably required in order to account for the data. The evolution of the spectral hardness is used as a sensitive indicator of the nature of the soft component, and of the possible contribution of an imbedded pulsar to the X-ray spectrum.
1988
Mastichiadis A, Oegelman H, Kirk JG. Spectral characteristics of the emerging X-rays from a possible pulsar in supernova 1987A. [Internet]. 1988;201:L19 - L22. WebsiteAbstract
We report on Monte-Carlo calculations of the emergence of X- and γ-rays from the envelope of the supernova 1987A. Assuming that there exists an X-ray emitting pulsar at the centre, we discuss the possibilities of observing its radiation and find that if the pulsar is about as luminous as the present-day Crab, it may be possible to detect the breakout at ~ 20 months. The pulsar should emerge first in the 6-16 keV band covered by GINGA. About six months later it would appear in the 20-45 keV band of MIR-HEXE. Because the optical depth of the envelope at breakout is still substantial (~4), we do not expect pulses to be detected straightaway. Our conclusions are based on a simple homogeneous model for the envelope and do not depend on whether or not ^56^Co is mixed into the outer layers. The time at which a pulsar of higher luminosity would emerge can be obtained from a simple scaling of our results.
1987
Mastichiadis A, Brecher K, Marscher AP. Electromagnetic Cascades in the Magnetosphere of a Very Young Pulsar: A Model for the Positron Production near the Galactic Center. [Internet]. 1987;314:88. WebsiteAbstract
A detailed model for positron production by a young pulsar is presented. It is shown that electromagnetic cascades can develop in a young pulsar's magnetosphere, and the model results are applied to the pulsar which is hypothesized to lie near the Galactic center. It is found that such a pulsar would be expected to produce relatively low energy electron-positron pairs with an efficiency rating high enough to explain the observed luminosity of the Galactic center annihilation line. Virtually all of the gamma ray continuum radiation produced in the cascades would be beamed along the magnetic poles of the neutron star, and therefore probably would not be observed from earth. Some observational predictions generated by the proposed model for the Galactic center positron source are given.
Hermsen W, Bennett K, Bloemen JBGM, Buccheri R, Jansen FA, Mastichiadis A, Mayer-Hasselwander HA, Özel ME, Pollock AMT, Strong AW. High-energy gamma-ray and hard X-ray observations of CYG X-3. [Internet]. 1987;175:141 - 150. WebsiteAbstract
High-energy (70 MeV < E < 5 GeV) gamma-ray observations of Cyg X-3 by the ESA satellite COS-B and hard X-ray (14 keV < E < 140 keV) observations by the Leiden-MIT balloon experiment, Leimit, are presented. A comparison with, and a study of earlier results over the 14 decades in energy from 1 keV up to ≡1017eV indicates that the strong variability of Cyg X-3 over more than one order of magnitude at energies below 20 keV does not exhibit itself in the data collected at hard X-ray energies, and the power emitted per decade of energy reaches a minimum in the MeV-GeV region. If the primary gamma-rays up to 1015eV originate close to the central source, absorption by the keV X-rays in the Cyg X-3 binary system could explain the latter phenomenon.
1986
Mastichiadis A. The implications of ultra-high-energy activity in X-ray binary systems. [Internet]. 1986;169:373 - 379. WebsiteAbstract
A mechanism for producing the high energy γ-ray spectrum of X-ray binaries exhibiting ultra-high-energy activity is proposed. γ-rays can result from decay of π0 produced in collisions of energetic protons with the keV ambient photons. Consequently, the X-ray photons modify the produced primary spectrum by selectively absorbing the lower energy part of it. As a result of this differential absorption these systems should exhibit a power law spectrum of index 1.5 - 2.0. Moreover, these systems should have a high electron-positron pair number density that will lead to 511 keV annihilation line emission.
Mastichiadis A, Marscher AP, Brecher K. Electron-Positron Pair Production by Ultrarelativistic Electrons in a Soft Photon Field. [Internet]. 1986;300:178. WebsiteAbstract
The fully differential cross section for photon-electron pair production is integrated numerically over phase space. Results are obtained for the astrophysically interesting case in which the interaction between an ultrarelativistic electron and a soft photon results in electron-positron pair production. The positron spectrum is a function of the energies of both the photon and the electron, as well as the angle of interaction. It is found that the energy at which the positron distribution peaks is inversely proportional to the photon energy and independent of the electron energy. The positron spectrum is integrated once more over initial electron energies for a power-law energy distribution of primary electrons. The same procedure is repeated for the recoil particle; it is shown that the peak of the recoil energy distribution depends linearly on the energy of the primary electron. Finally, semianalytical expressions are obtained for the energy losses of the primary electrons.
1985
Mastichiadis A. Electron-Positron Pair Production by Ultrarelativistic Electrons in a Soft Photon Field. [Internet]. 1985. WebsiteAbstract
The fully differential cross section for photon -electron pair production is integrated numerically over phase space. We obtain results for the astrophysically interesting case in which the interaction between an ultrarelativistic electron and a soft photon results in e('-) - e('+) pair production. The positron spectrum is a function of the energies of both the photon and the electron, as well as the angle of interaction. We integrate over solid angle by assuming an isotropic distribution of photons. We find that the energy at which the positron distribution peaks is inversely proportional to the photon energy and independent of the electron energy. The positron spectrum is integrated once more over initial electron energies for a power law energy distribution of primary electrons. The same procedure is repeated for the recoil particle; it is shown that the peak of the recoil energy distribution depends linearly on the energy of the primary electron. Finally, semi-analytical expressions are obtained for the energy losses of the primary electrons. Triplet pair production turns out to be important in the magnetospheres of hot (T (TURNEQ) 10('7) K), young (t (LESSTHEQ) 200 yrs) neutron stars. Interactions between thermal photons and highly relativistic electrons accelerated in the pulsar magnetosphere can initiate intense electromagnetic cascades. The development of such cascades is followed and some conclusions for the properties of young neutron stars are drawn. Finally, the possibility of a young pulsar (presumably arising from a recent supernova explosion) in the galactic center region is considered; it is shown that it can explain the intense positron production observed from this region, without conflicting other observations.