12-20 July 2017
Asia/Seoul timezone
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BEXCO - Room F(201/202/203/204)

[GA256] Extragalactic gamma-ray propagation: beyond the absorption-only model



Primary authors



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. We review the available models of extragalactic gamma-ray propagation with an emphasis on the so-called “cascade models” which account for the cascade process that produces secondary photons on the cosmic microwave background (CMB) and EBL. The cascade component may significantly contribute to the observed flux of some blazars below 1 TeV if the EGMF strength in voids of the large scale structure is smaller than 1 fG at the characteristic spatial scale 1 Mpc. Indeed, several blazar observations do indicate that the cascade component does contribute to the observed flux. The “electromagnetic cascade model”, in which gamma-rays are the primary particles, has the following spectral signatures: 1) a high-energy cutoff caused by the $gammagamma-> e^{+}e^{-}$ 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, motivated by the recently obtained indication for the angular correlation of Fermi LAT gamma-rays, IceCube neutrinos, and extreme-energy cosmic rays, we consider different species of the “hadronic cascade models”, in which protons and nuclei are the primary particles. These models have a number of difficulties, but, nevertheless, are not excluded. A detailed account of our calculations could be found in astro-ph/1609.01013 (accepted by A&A).