Detectability of ultrahigh energy cosmic ray signatures in gamma rays

Abstract

The injection of ultra-high 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. With respect to the gamma-ray flux associated to synchrotron radiation of ultra-high energy secondary pairs, we demonstrate that it does not depend strongly on these parameters and conclude that its magnitude ultimately depends on the energy injected in the primary cosmic rays. Bounding the cosmic ray luminosity with the contribution to the measured cosmic ray spectrum, we then find that the gamma-ray halo produced by equal luminosity sources is well below 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 ultra-high energy cosmic rays. This latter signal strongly depends 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 percents. Finally, we discuss briefly the case of nearby potential sources such as Centaurus A.

13 pages, 8 figures, submitted to A&A

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• Variable Gamma-ray Emission Induced by Ultra-High Energy Neutral Beams: Application to 4C +21.35
• The Gamma-Ray Window to Intergalactic Magnetism
• IMAGINE: A comprehensive view of the interstellar medium, Galactic magnetic fields and cosmic rays
• Cosmogenic gamma-rays and the composition of cosmic rays
• Constraints on the origin of ultra-high-energy cosmic rays from cosmogenic neutrinos and photons
• High-Energy Emission Induced by Ultra-High-Energy Photons as a Probe of Ultra-High-Energy Cosmic-Ray Accelerators Embedded in the Cosmic Web
• Constraining the Emissivity of Ultrahigh Energy Cosmic Rays in the Distant Universe with the Diffuse Gamma-ray Emission
• An upper limit on the cosmic-ray luminosity of individual sources from gamma-ray observations
• Acceleration and propagation of ultrahigh energy cosmic rays
• Synchrotron pair halo and echo emission from blazars in the cosmic web: application to extreme TeV blazars
• Gamma-ray counterparts of 2WHSP high-synchrotron-peaked BL Lac objects as possible signatures of ultra-high-energy cosmic-ray emission
• Non-variable cosmologically distant gamma-ray emitters as an imprint of propagation of ultra-high-energy protons