Radiation from the impact of broad-line region clouds onto AGN accretion disks

Abstract

Context. Active galactic nuclei are supermassive black holes surrounded by an accretion disk, two populations of clouds, bipolar jets, and a dusty torus. The clouds move in Keplerian orbits at high velocities. In particular, the broad-line region (BLR) clouds have velocities ranging from 1000 to 10000 km s −1 . Given the extreme proximity of these clouds to the supermassive black hole, frequent collisions with the accretion disk should occur. Aims. The impact of BLR clouds onto the accretion disk can produce strong shock waves where particles might be accelerated. The goal of this work is to investigate the production of relativistic particles, and the associated non-thermal radiation in these events. In particular, we apply the model we develop to the Seyfert galaxy NGC 1068. Methods. We analyze the efficiency of diffusive shock acceleration in the shock of colliding clouds of the BLR with the accretion disk. We calculate the spectral energy distribution of photons generated by the relativistic particles and estimate the number of simultaneous impacts needed to explain the gamma radiation observed by Fermi in Seyfert galaxies. Results. We find that is possible to understand the measured gamma emission in terms of the interaction of clouds with the disk if the hard X-ray emission of the source is at least obscured between 20% and 40%. The total number of clouds contained in the BLR region might be between 3 × 10 8 and 6 × 10 8 , which are values in good agreement with the observational evidence. The maximum energy achieved by the protons (∼ PeV) in this context allows the production of neutrinos in the observing range of IceCube.

High-Energy Neutrinos from NGC 1068

“…For the acceleration mechanisms entertain in [9][10][11], the column density cannot (significantly) surpass −1 otherwise collisions would control and largely reduce ,max . The neutrino flux predicted by the AGN-core model is about an order of magnitude larger than the estimate in [12], which is normalized to accommodate gamma-ray observations.…”

“…Observational studies and theoretical modeling are used to guide us in choosing the model parameters. Before proceeding, we pause to note that related ideas for modelling neutrino emission from NGC 1068 have been discussed in [9][10][11][12].…”

High-Energy Neutrinos from NGC 1068

“…For the acceleration mechanisms entertain in [9][10][11], the column density cannot (significantly) surpass σ −1 T otherwise pp collisions would control and largely reduce E p,max . The neutrino flux predicted by the AGN-core model is about an order of magnitude larger than the estimate in [12], which is normalized to accommodate gamma-ray observations.…”

“…Observational studies and theoretical modeling are used to guide us in choosing the model parameters. Before proceeding, we pause to note that related ideas for modelling neutrino emission from NGC 1068 have been discussed in [9][10][11][12].…”

Gamma-ray and Neutrino Signals from Accretion Disk Coronae of Active Galactic Nuclei

“…Several theoretical models have already been proposed to explain the neutrino excess seen in NGC 1068, such as accretion disk corona [17][18][19][20], the interaction of broad-line-region clouds with accretion disk [21], and galactic cosmic-ray halo [22]. However, as always, for theoretical models, model uncertainties in all of these models are still non-negligible.…”