Nonthermal Emission from Fall-back Clouds in the Broad-line Region of Active Galactic Nuclei

Abstract

The spectra of active galactic nuclei exhibit broad-emission lines that presumably originate in the broad-line region (BLR) with gaseous-dusty clouds in a predominantly Keplerian motion around the central black hole. Signatures of both inflow and outflow motion are frequently seen. The dynamical character of BLR is consistent with the scenario that has been branded as the failed radiatively accelerated dusty outflow. In this scheme, frequent high-velocity impacts of BLR clouds falling back onto the underlying accretion disk are predicted. The impact velocities depend mainly on the black hole mass, accretion rate, and metallicity, and they range from a few km s −1 up to thousands of km s −1 . Formation of strong shocks due to the collisions can give rise to the production of relativistic particles and associated radiation signatures. In this work, the nonthermal radiation generated in this process is investigated, and the spectral energy distributions for different parameter sets are presented. We find that the nonthermal processes caused by the impacts of clouds can lead to emission in the X-ray and the gamma-ray bands, playing the cloud density and metallicity key roles.