VHE Gamma-Ray Induced Pair Cascades in Blazars and Radio Galaxies: Application to NGC 1275

Abstract: Recent blazar detections by HESS, MAGIC, and VERITAS suggest that very-high-energy (VHE, E > 100 GeV) gamma-rays may be produced in most, if not all, types of blazars, including those that possess intense circumnuclear radiation fields. In this paper, we investigate the interaction of nuclear VHE gamma-rays with the circumnuclear radiation fields through gamma-gamma absorption and pair production, and the subsequent Compton-supported pair cascades. We have developed a Monte-Carlo code to follow the spatial development of the cascade in full 3-dimensional geometry, and calculate the radiative output due to the cascade as a function of viewing angle with respect to the primary VHE gamma-ray beam (presumably the jet axis of the blazar). 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 demonstrate that this scenario can explain the Fermi flux and spectrum of the radio galaxy NGC 1275.