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Differential spectra of electrons and positrons from photon-photon interactions for different values of s 0 : 10 (solid line), 3 (dashed line), 2 (dot-dashed line) and 1.5 (dotted line).  

Differential spectra of electrons and positrons from photon-photon interactions for different values of s 0 : 10 (solid line), 3 (dashed line), 2 (dot-dashed line) and 1.5 (dotted line).  

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We study the formation of giant electron-positron pair halos around the powerful high energy extragalactic sources. We investigate the dependence of radiation of pair halos, in particular the spectral and angular distributions on the energy spectrum of the primary gamma rays, the redshift of the source, and the flux of the extragalactic background...

Contexts in source publication

Context 1
... γ is the energy of the electron (positron) (in m e c 2 units) and ǫ = ε + ω ≈ ε is the photon energy in the lab-frame. The range of γ is Figure 2 shows the distributions of electrons (positrons) for different s 0 . ...
Context 2
... the epoch z = 2, the mean free paths of gamma-rays and electrons in the intergalactic medium were an order of magnitude shorter than at the present epoch. This should obviously result in significant shift of the SED towards low energies and more compact halos (in addition to the effects introduced by the redshift itself). This is demonstrated in Fig. 12, where the spectral luminosities and angular distributions of radiation of pair halos are shown for different redshifts of central sources. The total luminosity of primary gamma-rays is assumed 10 45 ...

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... A pair halo refers to the case where a strong IGMF (10 −12 -10 −9 G) isotropizes a large fraction of high-energy charged particles in the cascade, causing them to accumulate around the source. The reprocessed emission arriving at the observer is consequently extremely time-delayed, on the order of hundreds of years (Eungwanichayapant and Aharonian, 2009). ...
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... Brüggen et al. 2005;Brüggen 2013)-and thus the pairs are quickly isotropized. These are the typical conditions for the formation of giant pair halos (Aharonian et al. 1994;Eungwanichayapant & Aharonian 2009). However, studies of the point spread function (PSF) in the H.E.S.S. and VERITAS images of our targets seem to exclude the presence of extended emission (Abramowski et al. 2014;Archambault et al. 2017). ...
... However, studies of the point spread function (PSF) in the H.E.S.S. and VERITAS images of our targets seem to exclude the presence of extended emission (Abramowski et al. 2014;Archambault et al. 2017). Furthermore, the spectrum should be softer than observed below 200 GeV and the intrinsic primary flux is required to be ∼ 100× higher than the flux from EBL-correction alone, because of the re-isotropization (Eungwanichayapant & Aharonian 2009). This would increase dramatically the energy requirements of the source, both in luminosity and peak energy of the primary emission, making the problem of explaining the emitted SED even worse. ...
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... Brüggen et al. 2005;Brüggen 2013)-and thus the pairs are quickly isotropized. These are the typical conditions for the formation of giant pair halos (Aharonian et al. 1994;Eungwanichayapant & Aharonian 2009). However, studies of the point spread function (PSF) in the H.E.S.S. and VERITAS images of our targets seem to exclude the presence of extended emission (Abramowski et al. 2014;Archambault et al. 2017). ...
... However, studies of the point spread function (PSF) in the H.E.S.S. and VERITAS images of our targets seem to exclude the presence of extended emission (Abramowski et al. 2014;Archambault et al. 2017). Furthermore, the spectrum should be softer than observed below 200 GeV and the intrinsic primary flux is required to be ∼ 100× higher than the flux from EBL-correction alone, because of the re-isotropization (Eungwanichayapant & Aharonian 2009). This would increase dramatically the energy requirements of the source, both in luminosity and peak energy of the primary emission, making the problem of explaining the emitted SED even worse. ...
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Hard-TeV BL Lacs are a new type of blazars characterized by a hard intrinsic TeV spectrum, locating the peak of their gamma-ray emission in the spectral energy distribution (SED) above 2-10 TeV. Such high energies are problematic for the Compton emission, using a standard one-zone leptonic model. We study six examples of this new type of BL Lacs in the hard X-ray band with the NuSTAR satellite. Together with simultaneous observations with the SWIFT satellite, we fully constrain the peak of the synchrotron emission in their SED, and test the leptonic synchrotron self-Compton (SSC) model. We confirm the extreme nature of 5 objects also in the synchrotron emission. We do not find evidence of additional emission components in the hard X-ray band. We find that a one-zone SSC model can in principle reproduce the extreme properties of both peaks in the SED, from X-ray up to TeV energies, but at the cost of i) extreme electron energies with very low radiative efficiency, ii) conditions heavily out of equipartition (by 3 to 5 orders of magnitude), and iii) not accounting for the simultaneous UV data, which then should belong to a different emission component, possibly the same as the far-IR (WISE) data. We find evidence of this separation of the UV and X-ray emission in at least two objects. In any case, the TeV electrons must not "see" the UV or lower-energy photons, even if coming from different zones/populations, or the increased radiative cooling would steepen the VHE spectrum.
... Les nouveaux photons gamma diffusés peuvent alors être émis dans la direction de l'observateur. Ceci induit la formation d'une source étendue autour de la source ponctuelle (Aharonian et al. 1994;Eungwanichayapant and Aharonian 2009). ...
... La mesure dépend aussi de la bande d'énergie considérée. Des halos de paires doivent être observables sur les blazars comme sur les sursauts gamma dans différentes bandes d'énergie gamma (Eungwanichayapant and Aharonian 2009) et même potentiellement dans la bande X (1 -10 keV) (Coppi 1998). La complexité d'observer des halos réside aussi dans la capacité à pouvoir distinguer le halo lié à la cascade du bruit de fond. ...
... ◮ Eungwanichayapant and Aharonian (2009) propose un code qui considère que les leptons sont isotropisés avant interaction (c'est-à-dire émis dans toutes les directions) ce qui crée une zone de cascades étendue. Ceci est valide pour des amplitudes du champ magnétique extragalactique comprise entre 10 −12 G et 10 −7 G. Comme la zone où se développe la cascade est assez restreinte, le spectre du fond diffus extragalactique est considéré comme ne variant pas sur l'ensemble du développement de la cascade (pas de prise en compte de l'évolution cosmologique du spectre). ...
Thesis
Cette thèse vise à étudier le phénomène dit de « cascades électromagnétiques cosmologiques ». Ces cascadessont typiquement générées dans le milieu intergalactique par l’absorption de rayons gamma sur les photons du fond optique / UV et par la production de paires électron / positron associés. Ces leptons eux-mêmes interagissent avec les photons du fond diffus cosmologique via diffusion inverse Compton pour produire de nouveaux rayons gamma qui eux même peuvent s’annihiler, générant à partir d’un unique photon primaire toute une gerbe de photons et de particules secondaires. D’un point de vue observationnel, le développement de cette cascade introduit trois effets : une déformation du spectre à haute énergie, un retard temporel dans l’arrivée des rayons gamma et une extension de la taille apparente de la source.Les cascades électromagnétiques cosmologiques ont commencé à être étudiées dans les années soixante. Mais ce n’est qu’à partir des années 2010 avec l’arrivée du satellite Fermi (entre autres) et des observations dans la bande au GeV et au TeV que la discipline a explosé. Le phénomène est particulièrement important. D’une part il altère le spectre observé des sources rendant difficile la compréhension de la physique de ces dernières. D’autre part les cascades se développant dans le milieu extragalactique, elles sont très sensibles à la composition de ce dernier (fond diffus de photons, champ magnétique). Or ce milieu étant très ténu, il est difficile à étudier. Les cascades deviennent alors une formidable sonde pour accéder à sa compréhension et pouvoir en comprendre l’origine qui remonte au commencement de l’Univers.Pourtant les cascades cosmologiques sont un phénomène complexe faisant intervenir des interactions difficiles à modéliser (sections efficaces complexes) et le transport de particules dans un Univers en expansion (cosmologie). Face à cette complexité les expressions analytiques sont vite limitées et le passage au numérique devient inévitable. Dans le cadre de cette thèse un code de simulation Monte Carlo a donc été développé visant à reproduire aussi précisément que possible le phénomène des cascades. Ce code a été testé et validé en le confrontant aux expressions analytiques.Grâce à ce code, le rôle des différents paramètres physiques impactant le développement de la cascade a été étudié de manière systématique. Cette étude a permis de mieux comprendre la physique du phénomène. En particulier, l’impact des propriétés du milieu extragalactique (fond diffus extragalactique, champ magnétique extragalactique) sur les observables a été mis en évidence.Finalement, une seconde étude a été menée pour mesurer la contribution des cascades au fond gamma extragalactique. Des travaux récents montrent qu’une grande partie de l’émission diffuse à très haute énergie provient de sources ponctuelles non résolues (blazars en particulier). Ces sources gamma (résolues et non résolues) doivent en principe initier des cascades qui peuvent contribuer au fond diffus. En partant d’une modélisation de l’émission des blazars à différents redshifts, l’absorption et la contribution des cascades ont alors été calculées à l’aide du code Monte Carlo. Les résultats montrent que la contribution des cascades au fond gamma extragalactique pourrait violer les limites Fermi mais l’excès doit encore être confirmé.
... First, the source spectrum is altered because each high energy TeV photon is reprocessed into thousands of GeV photons (Protheroe 1986;Roscherr & Coppi 1998;Aharonian et al. 2002;Neronov & Vovk 2010). Second, due to the deflection of leptons by the EGMF, new gamma-rays are emitted along different lines of sight, so that a point source may appear as extended (Aharonian et al. 1994;Eungwanichayapant & Aharonian 2009). Third, as leptons are deflected, cascade photons travel a longer distance and arrive with a significant time delay, as compared to unabsorbed, primary photons (Kronberg 1995;Plaga 1995;Ichiki et al. 2008;Murase et al. 2008;Takahashi et al. 2008). ...
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TeV photons from extragalactic sources are absorbed in the intergalactic medium and initiate electromagnetic cascades. These cascades offer a unique tool to probe the properties of the universe at cosmological scales. We present a new Monte Carlo code dedicated to the physics of such cascades. This code has been tested against both published results and analytical approximations, and is made publicly available. Using this numerical tool, we investigate the main cascade properties (spectrum, halo extension, time delays), and study in detail their dependence on the physical parameters (extra-galactic magnetic field, extra-galactic background light, source redshift, source spectrum and beaming emission). The limitations of analytical solutions are emphasised. In particular, analytical approximations account only for the first generation of photons and higher branches of the cascade tree are neglected.
... A general model for the shape of cascade spectra was developed in Zdziarski (1988). A more recent model can be found in Eungwanichayapant & Aharonian (2009) and is depicted as the grey curve in Fig. 3. Although predictions at the high-energy end of the cascade strongly depend on the cutoff energy of the injection spectrum, an index of ∼2 is expected in the energy range just before the secondary flux drops rapidly. ...
... In the publication by Eungwanichayapant & Aharonian (2009), a study of the formation of PHs was conducted. In particular, the authors investigated the spectral and angular distributions of PHs in relation to the redshift of the central source, the spectral shape of the primary γ-rays and the flux of the EBL. ...
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Article
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