Gamma rays from cloud penetration at the base of AGN jets

TL;DR

This paper models gamma-ray emission resulting from interactions between dense clouds in AGN broad line regions and the jets, exploring conditions for detectability and variability in different AGN types.

Contribution

It introduces a hydrodynamical and spectral model for jet-cloud interactions in AGNs, predicting gamma-ray signatures and variability patterns.

Findings

Jet-cloud interactions can produce detectable gamma-ray emission.

Gamma-ray flux depends on jet power and cloud interaction frequency.

Variability timescales are linked to cloud lifetime within the jet.

Abstract

Dense and cold clouds seem to populate the broad line region surrounding the central black hole in AGNs. These clouds could interact with the AGN jet base and this could have observational consequences. We want to study the gamma-ray emission produced by these jet-cloud interactions, and explore under which conditions this radiation would be detectable. We investigate the hydrodynamical properties of jet-cloud interactions and the resulting shocks, and develop a model to compute the spectral energy distribution of the emission generated by the particles accelerated in these shocks. We discuss our model in the context of radio-loud AGNs, with applications to two representative cases, the low-luminous Centaurus A, and the powerful 3C 273. Some fraction of the jet power can be channelled to gamma-rays, which would be likely dominated by synchrotron self-Compton radiation, and show typical variability timescales similar to the cloud lifetime within the jet, which is longer than several hours. Many clouds can interact with the jet simultaneously leading to fluxes significantly higher than in one interaction, but then variability will be smoothed out. Jet-cloud interactions may produce detectable gamma-rays in non-blazar AGNs, of transient nature in nearby low-luminous sources like Cen A, and steady in the case of powerful objects of FR II type.