Extragalactic gamma-ray propagation: beyond the absorption-only model

Abstract

Observations of extragalactic sources in the high energy (HE, E>100 MeV) and very high energy (VHE, E>100 GeV) domains allow to search for axion-like particles (ALP) and constrain the extragalactic magnetic field (EGMF) parameters. Such studies have recently received much attention thanks to the advent of new instruments and the progress of the extragalactic background light (EBL) models. Observations of blazars indicate that an additional component from electromagnetic (EM) cascades contributes to the observed flux, allowing for a possibility that the EGMF strength in voids of the large scale structure is smaller than 1 fG on the characteristic spatial scale of 1 Mpc. We consider various extragalactic $\gamma$-ray propagation models with an emphasis on the “electromagnetic cascade model”, which assumes that primary particles are $\gamma$-rays, and has the following spectral signatures: 1) a high-energy cutoff caused by the $\gamma\gamma$→e$^{+}$e$^{−}$ absorption process 2) an “ankle” formed by the intersection of the primary and cascade components 3) a possible low-energy “magnetic cutoff”, 4) a possible low-energy “second ankle” at the second intersection of the primary and cascade components. The ankle may mimic the signature of the $\gamma$-ALP oscillation process. Finally, we consider various species of the “hadronic cascade model”, in which primary particles are protons and nuclei. This model has a number of difficulties, but, nevertheless, is not excluded.