Cosmogenic gamma rays and the composition of cosmic rays

Abstract

We discuss the prospects of detecting the sources of ultra-high energy (UHE) cosmic ray (CR) nuclei via their emission of cosmogenic γ-rays in the GeV to TeV energy range. These γ-rays result from electromagnetic cascades initiated by high energy photons, electrons and positrons that are emitted by CRs during their propagation in the cosmic radiation background and are independent of the simultaneous emission of γ-rays in the vicinity of the source. The corresponding production power by UHE CR nuclei (with mass number A and charge Z) is dominated by pion photo-production (∝ A) and Bethe-Heitler pair production (∝ Z 2 ). We show that the cosmogenic γ-ray signal from a single steady UHE CR source is typically more robust with respect to variations of the source composition and injection spectrum than the accompanying signal of cosmogenic neutrinos. We study the diffuse emission from the sum of extragalactic CR sources as well as the point source emission of the closest sources.

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

“…Unless assuming very soft spectral indices, expected fluxes of secondaries remain in the same order as in the proton case. A similar discussion can be found in Ahlers & Salvado (2011).…”

“…Above 10 19 eV the universe becomes more and more transparent to photons and very high-energy gamma-rays can propagate a few tens of megaparsecs without losing a great amount of energy. As a result, these very high-energy cosmogenic gamma-rays were discussed in the literature either as signatures of the so-called Top-Down models (see for instance Protheroe & Johnson 1996;Lee 1998;Sigl et al 1999;Semikoz & Sigl 2004) or as probes of UHECR acceleration in the local universe (e.g., Yoshida & Teshima 1993;Protheroe & Johnson 1996;Lee 1998;Semikoz & Sigl 2004;Gelmini et al 2007a,b;Taylor & Aharonian 2009;Kuempel et al 2009;Hooper et al 2011;Ahlers & Salvado 2011).…”

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).…”

“…Above 10 19 eV the universe becomes more and more transparent to photons and very high energy gamma rays can propagate a few tens of megaparsecs without losing a large amount of energy. As a result, these very high energy cosmogenic gamma-rays were discussed in the literature either as signatures of the so called Top-Down models (see for instance [166,167,168,169]) or as probes of UHECR acceleration in the local universe (e.g, [170,166,167,169,171,172,173,174,175,176,177,165]). Since electromagnetic cascades, piling up below 100 GeV, are produced during UHECR propagation and are likely to be associated with the production of cosmogenic neutrinos, it has been soon realized that measurements of the diffuse gamma-ray background could allow to put constraints on the cosmological evolution of the UHECR luminosity [23] and on the maximum allowable cosmogenic neutrino fluxes [178].…”

Synchrotron pair halo and echo emission from blazars in the cosmic web: application to extreme TeV blazars

“…Assuming that blazar emission is steady, Fermi/LAT data (or upper limits) lead to lower limits of B voids λ 1/2 < ∼ 10 −15 G Mpc 1/2 (e.g. Neronov & Vovk 2010;Taylor et al 2011;Ahlers & Salvado 2011;Vovk et al 2012). Considering the possibility that blazar emission is transient rather than steady, conservative constraints can be obtained, which are 10 −18 −10 −20 G Mpc 1/2 (e.g.…”