Gamma-rays from the IC $e^\pm$ pair cascade in the radiation field of an accretion disk: Application to CenA

TL;DR

This paper models gamma-ray spectra from inverse Compton e± pair cascades near accretion disks, applying it to explain TeV gamma-ray observations of Cen A and constraining the emission region location.

Contribution

It introduces a detailed cascade model considering anisotropic radiation fields and synchrotron losses, applied specifically to Cen A's gamma-ray emission.

Findings

The gamma-ray spectra depend strongly on observation angle.

The model successfully explains Cen A's TeV gamma-ray observations.

Constraints on the gamma-ray emission site in Cen A are derived.

Abstract

The very short time scale variability of TeV $\gamma$-ray emission from active galaxies suggests that the acceleration process of particles and the production of primary $\gamma$-rays likely occurs relatively close to the accretion disk. We calculate the $\gamma$-ray spectra produced in an Inverse Compton $e^\pm$ pair cascade initiated by primary $\gamma$-rays which are injected close to the surface of the accretion disk. Possible synchrotron energy losses of secondary cascade $e^\pm$ pairs are also taken into account. Since the soft radiation field is anisotropic, the resulting $\gamma$-ray spectra strongly depend on the observation angle. We investigate their basic properties for different parameters describing such a model. The model is applied to the misaligned blazar Cen A recently detected in the TeV $\gamma$-rays. We conclude on the site of the $\gamma$-ray emission region in Cen A based on the comparison of the model with the observations of this source in the GeV-TeV energy range.