ROLE OF LINE-OF-SIGHT COSMIC-RAY INTERACTIONS IN FORMING THE SPECTRA OF DISTANT BLAZARS IN TeV GAMMA RAYS AND HIGH-ENERGY NEUTRINOS

Abstract

Active galactic nuclei (AGN) can produce both gamma rays and cosmic rays. The observed high-energy gamma-ray signals from distant blazars may be dominated by secondary gamma rays produced along the line of sight by the interactions of cosmic-ray protons with background photons. This explains the surprisingly low attenuation observed for distant blazars, because the production of secondary gamma rays occurs, on average, much closer to Earth than the distance to the source. Thus the observed spectrum in the TeV range does not depend on the intrinsic gamma-ray spectrum, while it depends on the output of the source in cosmic rays. We apply this hypothesis to a number of sources and, in every case, we obtain an excellent fit, strengthening the interpretation of the observed spectra as being due to secondary gamma rays. We explore the ramifications of this interpretation for limits on the extragalactic background light and for the production of cosmic rays in AGN. We also make predictions for the neutrino signals, which can help probe the acceleration of cosmic rays in AGN.

Cosmogenic gamma rays and the composition of cosmic rays

“…However, it is apparent that this model does not necessarily require the presence of an IGMF to be compatible with the Fermi-LAT limit as already noted by Ref. [57]. We also show in this case the reduction of the PS signal via the presence of an IGMF with coherence length λ B = 1 Mpc and a strength B 0 = 10 −16 G, 10 −15 G and 10 −14 G, respectively.…”

“…This possibility has been previously studied for proton sources in Refs. [51][52][53][54] and has been revived recently in the context of unusually bright though distant TeV γ-ray sources [55][56][57]. Here we extend the discussion to the case of UHE CR nuclei and study the effect of IGMFs on the observability of the PS flux in detail.…”

“…This model has been advocated in Refs. [55][56][57]. In this case the observed spectrum is assumed to be dominated by cosmogenic γ-rays emitted during propagation.…”

Non-variable cosmologically distant gamma-ray emitters as a propagation imprint of ultra-high-energy protons

“…However, the SEDs of the radiations produced through these two processes are different. The SED of the cascade radiation has its maximum at higher energies, and the slope before (after) maximum is flatter (steeper) than the slope of the SED of the synchrotron radiation (see, e.g., Taylor et al 2011;Essey et al 2011). …”

“…Formally, these weak intergalactic magnetic fields cannot be excluded. Moreover, it has been argued recently that very high-energy tails of gamma-ray spectra of distant TeV blazars might be contributed by cascades initiated by UHE protons in the intergalactic medium with a magnetic field as weak as 10 −15 G (Essey et al 2011).…”