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

Abstract

The very short time scale variability of TeV $γ$-ray emission from active galaxies suggests that the acceleration process of particles and the production of primary $γ$-rays likely occurs relatively close to the accretion disk. We calculate the $γ$-ray spectra produced in an Inverse Compton $e^\pm$ pair cascade initiated by primary $γ$-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 $γ$-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 $γ$-rays. We conclude on the site of the $γ$-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.