Detectability of ultrahigh energy cosmic-ray signatures in gamma-rays

Abstract

The injection of ultrahigh energy cosmic-rays in the intergalactic medium leads to the production of a GeV-TeV gamma-ray halo centered on the source location, through the production of a high electromagnetic component in the interactions of the primary particles with the radiation backgrounds. This paper examines the prospects for the detectability of such gamma-ray halos. We explore a broad range of astrophysical parameters, including the inhomogeneous distribution of magnetic fields in the large-scale structure, as well as various possible chemical compositions and injection spectra; and we consider the case of a source located outside clusters of galaxies. We demonstrate that the gamma-ray flux associated to synchrotron radiation of ultrahigh energy secondary pairs does not depend strongly on these parameters, and conclude that its magnitude ultimately depends on the energy injected into the primary cosmic-rays. We find that the gamma-ray halo produced by equal luminosity sources (with cosmic ray luminosity and source density chosen to reproduce the measured cosmic-ray spectrum) is far fainter than current or planned instrument sensitivities. Only rare and powerful steady sources, located at distances larger than several hundreds of Mpc and contributing to a fraction 10% of the flux at 10 19 eV might be detectable. We also discuss the gamma-ray halos that are produced by inverse Compton/pair production cascades seeded by ultrahigh energy cosmic-rays. This depends strongly on the configuration of the extragalactic magnetic fields; it is dominated by the synchrotron signal on a degree scale if the filling factor of magnetic fields with B 10 −14 G is smaller than a few percent. Finally, we briefly discuss the case of nearby potential sources such as Centaurus A.

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.…”

Constraints on the origin of ultra-high-energy cosmic rays from cosmogenic neutrinos and photons

“…This possibility was also recently studied in Kotera et al (2011). In both studies, a source at 1 Gpc of luminosity 10 46 erg s −1 above 10 19 eV is found to possibly yield detectable fluxes for Fermi or CTA (for instance, a flux of 10 −9 GeV cm −2 s −1 spread over ∼0.2 deg).…”

“…In both studies, a source at 1 Gpc of luminosity 10 46 erg s −1 above 10 19 eV is found to possibly yield detectable fluxes for Fermi or CTA (for instance, a flux of 10 −9 GeV cm −2 s −1 spread over ∼0.2 deg). These synchrotron images are promising for the detection of distant UHECR accelerators and have the advantage that they are a signature of the pion production mechanism (see discussions in Gabici & Aharonian 2005;and Kotera et al 2011), unlike cascades in the case of distant sources (see below for the local universe). However, their signal is weaker in the TeV range than in the GeV, although their detection with CTA should still be possible.…”

Extragalactic propagation of ultrahigh energy cosmic-rays

“…The second type of gamma-ray signals could be provided by synchrotron radiation, from the pion decay and pair production induced UHE secondary pairs, in the magnetized environment of a source [191,192,193]. 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).…”