Gamma-ray signatures from pair cascades in recombination-line radiation fields

TL;DR

This paper models gamma-ray spectra from pair cascades in blazar jets interacting with recombination-line photons, explaining observed spectral features and escape mechanisms in sources like Markarian 501 and 3C 279.

Contribution

It introduces a coupled kinetic equation model for gamma-ray pair cascades considering recombination-line photon fields, providing new insights into spectral features and gamma-ray escape in blazars.

Findings

Spectral features at ~3 TeV in Markarian 501 explained by line photon up-scattering.

Troughs in 3C 279 spectrum coincide with pair production thresholds.

Gamma rays in 3C 279 escape from the broad emission line region.

Abstract

Beams of ultra-relativistic electrons in blazar jets develop pair cascades interacting with ambient soft photons. Employing coupled kinetic equations with escape terms, we model the unsaturated pair cascade spectrum. We assume that the gamma rays predominantly scatter off recombination-line photons from clouds photoionised by the irradiation from the accretion disk and the jet. The cascade spectrum is rather insensitive to the injection of hard electron spectra associated with the short-time variability of blazars. Adopting physical parameters representative of Markarian 501 and 3C 279, respectively, we numerically obtain spectral energy distributions showing distinct features imprinted by the recombination-line photons. The hints for a peculiar feature at ~3 TeV in the spectrum of Markarian 501, detected with the MAGIC telescopes during a strong X-ray flux activity in 2014 July, can be explained in this scenario as a result of the up-scattering of line photons by beam electrons and the low pair-creation optical depth. Inspecting a high-fidelity Fermi-LAT spectrum of 3C 279 from January 2018 reveals troughs in the spectrum that coincide with the threshold energies for gamma rays producing pairs in collisions with recombination-line photons, and the absence of exponential attenuation. Our finding implies that the gamma rays in 3C 279 escape from the edge of the broad emission line region.