Very high energy gamma rays from active galactic nuclei: Cascading on the cosmic background radiation fields and the formation of pair halos

Abstract

Recent high energy gamma-ray observations (E>100 MeV) of blazar AGN show emission spectra with no clear upper energy cutoff. AGN, considered to be possible sources for the highest energy cosmic rays, may have emission extending well into the VHE (very high energy, E>100 GeV) domain. Because VHE gamma-rays are absorbed by pair production on the intergalactic background radiation, much of this emission is not visible. Electromagnetic cascades initiated by absorbed VHE gamma-rays, however, may be observable. Since the velocities of e+e- pairs produced in a cascade are likely to be isotropized by an ambient random magnetic field, extended ``halos'' (R>1 Mpc) of pairs will be formed around AGN with VHE emission. Cascade radiation from these pair halos is emitted isotropically and is observable below a few TeV. The halo radiation can be distinguished by its characteristic variation in spectrum and intensity with angular distance from the central source. This variation depends weakly on the details of the central source model, e.g., the orientation and beaming/opening angle of an emitting jet. Limiting or determining the intensity of the pair halo can thus serve as a model-independent bound on or measure of the VHE power of AGN. Next-generation Cherenkov telescopes may be able to image a pair halo.