Non-thermal emission from fall-back clouds in the Broad-Line Region of Active Galactic Nuclei

TL;DR

This paper investigates how collisions of fall-back clouds in the Broad-Line Region of Active Galactic Nuclei produce non-thermal X-ray and gamma-ray emission, influenced by cloud properties and metallicity.

Contribution

It introduces a model for non-thermal emission resulting from cloud impacts in the BLR, highlighting the role of shock-accelerated particles in AGN spectra.

Findings

Non-thermal emission can reach X-ray and gamma-ray energies.

Impact velocities depend on black hole mass, accretion rate, and metallicity.

Cloud impacts significantly influence high-energy emission in AGNs.

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 (FRADO; Czerny & Hryniewicz 2011). 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 non-thermal radiation generated in this process is investigated, and the spectral energy distributions for different parameter sets are presented. We find that the non-thermal 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 a key role.