Gamma rays from cloud penetration at the base of AGN jets

Abstract

<i>Context</i>. Dense and cold clouds seem to populate the broad line region surrounding the central black hole in AGNs. These clouds may interact with the AGN jet base which could have observational consequences. <i>Aims</i>. We study the gamma-ray emission produced by these jet-cloud interactions, and explore the conditions under which this radiation would be detectable. <i>Methods.</i> 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. <i>Results</i>. Some fraction of the jet power may be channelled into gamma-ray energy, which is likely to be dominated by synchrotron self-Compton radiation, and have 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. <i>Conclusions</i>. Jet-cloud interactions may produce detectable gamma-rays in non-blazar AGNs that are transient in nearby low-luminous sources such as Cen A, and steady in the case of powerful objects of FR II type.