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.
We report a measurement of intergalactic magnetic fields using combined data from Atmospheric Cherenkov Telescopes and Fermi Gamma-Ray Space Telescope, based on the spectral data alone. If blazars are assumed to produce both gamma rays and cosmic rays, the observed spectra are not sensitive to the intrinsic spectrum of the source, because, for a distant blazar, secondary photons produced in line-of-sight cosmic-ray interactions dominate the signal. In this case, we find 0.01 fG < B < 30 fG. If one excludes the cosmic-ray component, the 0.01 fG lower limit remains, but the upper limit depends on the spectral properties of the source. We present the allowed ranges for a variety of model parameters.Comment: 13 pages, 3 figure
Gamma-ray telescopes have reported some surprising observations of multi-TeV photons from distant active galactic nuclei (AGN), which show no significant attenuation due to pair production on either the extragalactic background light (EBL), or the photons near the source. We suggest a new interpretation of these observations, which is consistent with both the EBL calculations and the AGN models. Cosmic rays with energies below 50 EeV, produced by AGN, can cross cosmological distances, interact with EBL relatively close to Earth, and generate the secondary photons observed by γ-ray telescopes. We calculate the spectrum of the secondary photons and find that it agrees with the γ-ray data. The delays in the proton arrival times can explain the orphan flares, the lack of time correlations, and the mismatch of the variability time scales inferred from the multiwavelength observations. The γ-ray data are consistent with the detection of the secondary photons, which has important ramifications for gamma-ray astronomy, cosmic ray physics, EBL, and the intergalactic magnetic fields (IGMF).
Secondary photons and neutrinos produced in the interactions of cosmic ray protons emitted by distant active galactic nuclei (AGN) with the photon background along the line of sight can reveal a wealth of new information about the intergalactic magnetic fields, extragalactic background light, and the acceleration mechanisms of cosmic rays. The secondary photons may have already been observed by gamma-ray telescopes. We show that the secondary neutrinos improve the prospects of discovering distant blazars by IceCube, and we discuss the ramifications for the cosmic backgrounds, magnetic fields, and AGN models.
The observed very high energy spectra of distant blazars are well described by secondary gamma rays produced in line-of-sight interactions of cosmic rays with background photons. In the absence of the cosmic-ray contribution, one would not expect to observe very hard spectra from distant sources, but the cosmic ray interactions generate very high energy gamma rays relatively close to the observer, and they are not attenuated significantly. The same interactions of cosmic rays are expected to produce a flux of neutrinos with energies peaked around 1 PeV. We show that the diffuse isotropic neutrino background from many distant sources can be consistent with the neutrino events recently detected by the IceCube experiment. We also find that the flux from any individual nearby source is insufficient to account for these events. The narrow spectrum around 1 PeV implies that some active galactic nuclei can accelerate protons to EeV energies.PACS numbers: 95.85. Ry,98.70.Sa,98.54.Cm The IceCube collaboration has detected two neutrinos with energies 1.04 ± 0.16 and 1.14 ± 0.17 PeV [1,2]. These neutrinos are either electron or tau neutrinos. The muon analysis, currently under way, is expected to produce additional events (probably, with a lower energy resolution). The narrow energy range in which the two neutrinos have been detected may be consistent with a spectrum peaked in the PeV energy range, above the experimental threshold of 0.4 PeV and below the Glashow resonance that enhances detector sensitivity around 6.3 PeV [3]. Only specific types of astrophysical sources can produce a peaked spectrum around a PeV [4]. Narrow spectra peaked around 1 PeV were predicted to arise from line-of-sight interactions of cosmic rays emitted by blazars [5][6][7]. There is growing evidence that intergalactic cascades initiated by line-of-sight interactions of cosmic rays produced by active galactic nuclei (AGNs) are responsible for the highest-energy gamma rays observed from blazars [5][6][7][8][9][10][11][12][13]. As long as the intergalactic magnetic fields are in the femtogauss range [14], the spectra of distant blazars are explained remarkably well with secondary photons from such cascades [5][6][7]. In the absence of such contribution, some unusually hard intrinsic spectra [15][16][17] or hypothetical new particles [18] have been invoked to explain the data. Models for hard intrinsic spectra of γ rays can be constructed, but the natural ease with which secondary photons reproduce the data makes the explanation based on cosmic rays very appealing. Furthermore, the lack of time variability of the most distant blazars at energies above TeV is in agreement with this hypothesis, which predicts that the shortest variability time scales for z > ∼ 0.15 and E > ∼ 1 TeV should be greater than (0.1 − 103 ) years, depending on the model parameters [11].Proton acceleration in relativistic shocks is determined by the shock Lorentz factor, the magnetization of the preshock flow, and the orientation of the field relative to the shock propagati...
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