"Flat" broad line region and gamma-ray absorption in blazars

F. Tavecchio, G. Ghisellini

Published 2012-09-11Version 1

We study the impact of the geometry of the broad line region (BLR) on the expected absorption, through the \gamma\gamma\rightarrow e^{\pm} process, of gamma rays produced in the relativistic jet of flat spectrum radio quasars (FSRQ). We consider "flat" (or "disky") BLR models, and use BLR spectra calculated with the photoionization code CLOUDY, already used to investigate the emission and the absorption of high-energy photons in FSRQ. We characterize the energy-dependent optical depth of the process, \tau(E), for different accretion disk luminosities, aperture angles of the BLR (\alpha, as measured from the equatorial plane), and initial injection eighths of the high-energy photons, $R_{\rm o}$. We study in particular how the change of these parameters influences the spectral break at GeV energies, predicted if the emission occurs within the BLR. We found a well defined relation between the break energy and the post-break slope, both uniquely determined by \alpha. We finally find that even a rather disk-like BLR (\alpha \sim 25^\circ) corresponds to important absorption (\tau>1) of photons above few tens of GeV produced within the BLR. We therefore conclude that the VHE emission detected from FSRQs occurs beyond the BLR.