Secondary Photons and Neutrinos from Cosmic Rays Produced by Distant Blazars

Abstract

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.

Cosmogenic gamma rays and the composition of cosmic rays

“…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.…”

Extragalactic propagation of ultrahigh energy cosmic-rays

“…In these cases, the detection is most promising for extremely powerful sources located around 1 Gpc, which are less constrained by their contribution to the UHECR flux and would keep an angular size well below 1 degree [194]. Interestingly, GeV-TeV cascades were recently proposed in [195] (and recently re-examined in [177]) as a possible interpretation of the TeV signal observed by HESS in the direction of the AGN 1ES0229+200 (z=0.14). Assuming a point source cascade signal, Essey et al estimated the implied source luminosity in cosmic ray protons above 10 16 eV to be between ∼ 10 46 and 10 49 erg s −1 (depending on the maximum energy assumed).…”

“…Very powerful sources could also produce large point-like (or quasi pointlike), fluxes of cosmogenic neutrinos (see recently [195,165]) due to the fact that, above the pion production threshold with CMB photons, most of the interactions take place within say 10-20 Mpc from the source. Neutrino fluxes reaching 10 −8 GeV cm −2 s −1 (which might be detectable during IceCube operating time) at ∼ 10 18 eV could be produced by a source of luminosity L CR = 10 47 erg s −1 located at 1 Gpc (see Fig.…”

Understanding the spectrum of a distant blazar PKS 1424+240 and its implications

“…At the higher end of the spectrum, gamma rays must interact with extragalactic background light (EBL) and lose energy through pair production. However, line-of-sight interactions of cosmic rays with cosmic microwave background radiation (CMBR) and EBL can generate secondary gamma rays relatively close to the observer, which provides a plausible explanation of the observed hard spectra [1][2][3][4][5][6][7][8][9][10][11][12][13]. Primary gamma rays dominate the observed radiation from nearby blazars, and only the more distant sources provide an opportunity to study secondary gamma rays.…”