Publications by Year: 2014

Petropoulou M. The role of hadronic cascades in GRB models of efficient neutrino production. [Internet]. 2014;442:3026 - 3036. WebsiteAbstract
We investigate the effects of hadronic cascades on the gamma-ray burst (GRB) prompt emission spectra in scenarios of efficient neutrino production. By assuming a fiducial GRB spectrum and a power-law proton distribution extending to ultrahigh energies, we calculate the proton cooling rate and the neutrino emission produced through photopion processes. For this, we employ a numerical code that follows the formation of the hadronic cascade by taking into account non-linear feedback effects, such as the evolution of the target photon field itself due to the contribution of secondary particles. We show that in cases of efficient proton cooling and subsequently efficient high-energy neutrino production, the emission from the hadronic cascade distorts and may even dominate the GRB spectrum. Taking this into account, we constrain the allowable values of the ratio ηp = Lp/Lγ, where Lp and Lγ are the isotropic equivalent proton and prompt gamma-ray luminosities. For the highest value of ηp that does not lead to the dominance of the cascading emission, we then calculate the maximum neutrino luminosity from a single burst and show that it ranges between (0.01-0.6)Lp and (0.5-1.4)Lγ for various parameter sets. We discuss possible implications of other parameters, such as the magnetic field strength and the shape of the initial gamma-ray spectrum, on our results. Finally, we compare the upper limit on ηp derived here with various studies in the field, and we point out the necessity of a self-consistent treatment of the hadronic emission in order to avoid erroneously high neutrino fluxes from GRB models.
Petropoulou M, Dimitrakoudis S, Mastichiadis A. Neutrino and Uhecr Spectra from MRK 421. In: Vol. 28. ; 2014. pp. 1460206. WebsiteAbstract
We present the neutrino and UHECR spectra obtained from a detailed fitting of the spectral energy distribution (SED) of Mrk 421 (March 2001) using two variations of the leptohadronic model. In particular, while the low-energy component (optical to X-rays) of the SED is fitted by synchrotron emission of primary electrons in both models, the high-energy one (GeV-TeV gamma-rays) is synchrotron emission attributed either to ultra-high energy protons (LHs model) or to secondary electrons produced by the decay of charged pions (LHπ model). In the LHπ case we find that the produced neutrino spectra are sharply peaked at Eν 30 PeV with a peak flux slightly below the IC-40 sensitivity limit for Mrk 421. In the LHs model, on the other hand, the neutrino spectra fall well outside the PeV energy range, but the calculated E 30 EeV — UHECR flux at earth is close to that observed by HiresI, Telescope Array and Pierre Augere experiments.
Dimitrakoudis S, Petropoulou M, Mastichiadis A. Self-consistent neutrino and UHE cosmic ray spectra from Mrk 421. [Internet]. 2014;54:61 - 66. WebsiteAbstract
We examine the neutrino and cosmic ray spectra resulting from two models of fitting the spectral energy distribution (SED) of the blazar Mrk 421 using a self-consistent leptohadronic code. The γ -ray emission is attributed to either synchrotron radiation of ultra-high energy protons (LHs model) or to synchrotron radiation from electrons that result from photopion interactions of lower energy protons (LH π model). Although both models succeed in fitting satisfactorily the SED, the parameter values that they use result in significantly different neutrino and cosmic-ray spectra. For the LH π model, which requires high proton energy density, we find that the neutrino spectrum peaks at an energy Eν,peak = 3.3 PeV which falls well within the energy range of recent neutrino observations. While at the same time its peak flux is just under the sensitivity limit of IC-40 observations, it cannot produce ultra-high energy cosmic rays. In the LHs model, on the other hand, neutrinos are far from being detectable because of their low flux and peak energy at Eν,peak ≃ 100 PeV. However, the propagation of protons produced by the decay of escaping neutrons results in an ultra-high energy cosmic ray flux close to that observed by Pierre Augere, HiRes and Telescope Array at energies Ep ≃ 30 EeV.
Petropoulou M, Lefa E, Dimitrakoudis S, Mastichiadis A. One-zone synchrotron self-Compton model for the core emission of Centaurus A revisited. [Internet]. 2014;562:A12. WebsiteAbstract
Aims: We investigate the role of the second synchrotron self-Compton (SSC) photon generation to the multiwavelength emission from the compact regions of sources that are characterized as misaligned blazars. For this, we focus on the nearest high-energy emitting radio galaxy Centaurus A and we revisit the one-zone SSC model for its core emission. Methods: We have calculated analytically the peak luminosities of the first and second SSC components by first deriving the steady-state electron distribution in the presence of synchrotron and SSC cooling, and then by using appropriate expressions for the positions of the spectral peaks. We have also tested our analytical results against those derived from a numerical code where the full emissivities and cross-sections were used. Results: We show that the one-zone SSC model cannot account for the core emission of Centaurus A above a few GeV, where the peak of the second SSC component appears. We thus propose an alternative explanation for the origin of the high-energy (≳0.4 GeV) and TeV emission, where these are attributed to the radiation emitted by a relativistic proton component through photohadronic interactions with the photons produced by the primary leptonic component. We show that the required proton luminosities are not extremely high, i.e. ~1043 erg/s, provided that the injection spectra are modelled by a power law with a high value of the lower energy cutoff. Finally, we find that the contribution of the core emitting region of Cen A to the observed neutrino and ultra-high-energy cosmic-ray fluxes is negligible.
Petropoulou M, Giannios D, Dimitrakoudis S. Implications of a PeV neutrino spectral cut-off in gamma-ray burst models. [Internet]. 2014;445:570 - 580. WebsiteAbstract
The recent discovery of extragalactic PeV neutrinos opens a new window to the exploration of cosmic ray accelerators. The observed PeV neutrino flux is close to the Waxman-Bahcall upper bound implying that gamma-ray bursts (GRBs) may be the source of ultrahigh energy cosmic rays (UHECRs). Starting with the assumption of the GRB-UHECR connection, we show using both analytical estimates and numerical simulations that the observed neutrinos can originate at the jet as a result of photopion interactions with the following implications: the neutrino spectra are predicted to have a cut-off at energy ≲10 PeV; the dissipation responsible for the GRB emission and cosmic ray acceleration takes place at distances rdiss ≃ 3 × 1011-3 × 1013 cm from the central engine; the Thomson optical depth at the dissipation region is τT ∼ 1; the jet carries a substantial fraction of its energy in the form of Poynting flux at the dissipation region, and has a Lorentz factor Γ ≃ 100-500. The non-detection of PeV neutrinos coincident with GRBs will indicate that GRBs are either poor cosmic accelerators or the dissipation takes place at small optical depths in the jet.
Petropoulou M, Dimitrakoudis S, Mastichiadis A, Giannios D. Hadronic supercriticality as a trigger for γ-ray burst emission. [Internet]. 2014;444:2186 - 2199. WebsiteAbstract
We explore a one-zone hadronic model that may be able to reproduce γ-ray burst (GRB) prompt emission with a minimum of free parameters. Assuming only that GRBs are efficient high-energy proton accelerators and without the presence of an ab initio photon field, we investigate the conditions under which the system becomes supercritical, i.e. there is a fast, non-linear transfer of energy from protons to secondary particles initiated by the spontaneous quenching of proton-produced γ-rays. We first show analytically that the transition to supercriticality occurs whenever the proton injection compactness exceeds a critical value, which favours high proton injection luminosities and a wide range of bulk Lorentz factors. The properties of supercriticality are then studied with a time-dependent numerical code that solves concurrently the coupled equations of proton, photon, electron, neutron and neutrino distributions. For conditions that drive the system deep into the supercriticality, we find that the photon spectra obtain a Band-like shape due to Comptonization by cooled pairs and that the energy transfer efficiency from protons to γ-rays and neutrinos is high reaching ∼0.3. Although some questions concerning its full adaptability to the GRB prompt emission remain open, supercriticality is found to be a promising process in that regard.
Petropoulou M. Time-dependent modelling of PKS 2155-304 in a low state. [Internet]. 2014;571:A83. WebsiteAbstract
Aims: We apply both leptonic and leptohadronic emission scenarios for modelling the multiwavelength photon spectra and the observed variability in the optical, X-ray, and TeV gamma-ray energy bands of blazar PKS 2155-304 while being in a low state between 25 August and 6 September 2008. Methods: We consider three emission models, namely a one-component synchrotron self-Compton model (1-SSC), a one-zone proton synchrotron model (LHs), and a two-component SSC model (2-SSC). Only in the first scenario can the emission from the optical up to very high-energy (VHE) gamma-rays be attributed to a single particle population from one emission region. In the LHs model, the low-energy and high-energy bumps of the spectral energy distribution (SED) are the result of electron and proton synchrotron radiation, respectively, i.e. two different particle populations are required. In the 2-SSC model, the emission from one component dominates in the optical and gamma-ray energy bands, while the other one contributes only to the X-ray flux. Using a time-dependent numerical code that solves the kinetic equations for each particle species, we derived, in all cases, acceptable fits to the time-averaged SED. By imposing variations to one (or more) model parameters according to observed variability pattern in one (or more) frequencies we calculated the respective lightcurves and compared them with the observations. Results: We show that the 1-SSC model cannot account for the anticorrelation observed between the X-rays and VHE gamma-rays, although it can explain the time-averaged SED. The anticorrelation can be more naturally explained by the two-component emission models. Both of them reproduce satisfactorily the optical, X-ray, and TeV variability but at the cost of additional free parameters, which from four in the 2-SSC model increase to six in the LHs model. Although the results of our time-resolved analysis do not favour one of the aforementioned models, they suggest that a two-component scenario is more adequate for the emission of PKS 2155-304 in the low state of 2008, which agrees with a recent independent analysis. This suggests that the quiescent blazar radiation might result from a superposition of the radiation from different components, while a flare might still be the result of a single component.
Petropoulou M. High-Energy Signatures from Leptohadronic Interactions in GRB Models. In: ; 2014. pp. 12. WebsiteAbstract
Gamma-ray bursts (GRB) have been long considered to be the sources of ultra high energy cosmic rays. If GRB jets are, indeed, sites of proton acceleration at high energies, then photohadronic processes, i.e. interactions between protons and photons, become relevant. In this talk, I will discuss some of their consequences for GRB models. First, I will present how we can constrain the physical conditions of the GRB emitting region by using indirect information from the copious neutrino emission that is naturally produced via photohadronic interactions on an ad-hoc Band photon spectrum. Second, I will present a model for the formation of Band-like photon spectra from first principles. This has been built on a recently discovered radiative instability, known as "spontaneous photon quenching." I will show that for a wide parameter range the instability sets in and establishes an efficient energy transfer from protons to secondaries produced through photohadronic interactions. It is then the interplay between photons and secondary electron-positron pairs, through purely leptonic processes, that actually determines the shape of the gamma-ray spectrum at steady-state.