Evidence for the Secondary Emission as the Origin of Hard Spectra in TeV Blazars

TL;DR

This paper proposes a model where both primary and secondary gamma-ray photons, produced within and outside the source respectively, explain the unexpectedly hard spectra observed in distant TeV blazars, especially 1ES 1101-232.

Contribution

The study introduces a combined model of primary SSC and secondary proton-induced gamma-ray production to explain hard spectra in distant blazars, supported by fitting observational data.

Findings

Secondary gamma-rays account for low attenuation in observed spectra.

The model fits well with the observed spectra of 1ES 1101-232.

Proton interactions outside the source explain the spectral shape.

Abstract

We develop a model for a possible origin of hard very high energy spectra from a distant blazar. In the model, both the primary photons produced in the source and secondary photons produced outside the source contribute to the observed high energy $\gamma$-rays emission. That is, the primary photons are produced in the source through the synchrotron self-Compton (SSC) process, and the secondary photons are produced outside the source through high energy protons interaction with the background photons along the line of sight. We apply the model to a characteristic case was the very high energy (VHE) $\gamma$-ray emissions in distant blazar 1ES 1101-232. Assuming a suitable electron and proton spectra, we obtain excellent fits to observed spectra of distant blazar 1ES 1101-232. This indicated that the surprisingly low attenuation of high energy $\gamma$-rays, especially for the shape of the very high energy $\gamma$-rays tail of the observed spectra, can be explained by secondary $\gamma$-rays produced in interactions of cosmic-ray protons with background photons in the intergalactic space.