Very-High-Energy Gamma-Ray Induced Pair Cascades in Blazars and Radio Galaxies

Recent blazar detections by H.E.S.S., MAGIC, and VERITAS suggest that very-high-energy (VHE, E > 100 GeV) gamma-ray production may be common to most radio-loud Active Galactic Nuclei (AGN). We have investigated the signatures of Compton-supported pair cascades initiated by VHE gamma-ray absorption in arbitrary radiation fields, including monochromatic Ly-alpha line emission from the Broad Line Region and thermal infrared radiation from a circum-nuclear dust torus. We follow the spatial development of the cascade in full 3-dimensional geometry. We show that even for relatively weak magnetic fields, the cascades can be efficiently isotropized, leading to substantial off-axis cascade emission peaking in the Fermi energy range at detectable levels for nearby radio galaxies. We provide model fits to the gamma-ray loud radio galaxies NGC 1275 and Cen A, demonstrating that off-axis cascade emission may play an important role for the gamma-ray emission from radio galaxies observed by Fermi.

This work was supported by NASA through Fermi Guest Investigator Grants NNX09AT81G and NNX10AO49G.