X/gamma-ray emission from hot accretion flows in AGNs

Abstract

We present preliminary results of our study of the impact of strong gravity effects on properties of the high energy radiation produced in accretion flows around supermassive black holes. We refine a model of the X-ray emission from a hot optically-th in flow by combining a fully general-relativistic (GR) hydrodynamical description of the flow with a fully GR description of Comptonization. We find that emission from a flow around a rapi dly rotating black hole is dominated by radiation produced within the innermost few gravitational radii, the region where effects of the Kerr metric are strong. The X-ray spectrum from such a fl ow depends on the inclination angle of the line of sight to the symmetry axis, θobs, with higher θobs characterised by a harder slope and a higher cut-off energy. For a non-rotating black hole, dependence on θobs is insignificant. Under the (reasonable) assumption that the equatorial plane of a rotating supermassive black hole is aligned with the surrounding torus, these predicted properties may provide a crucial extension of the unified model of AGNs, allowing to reconcile the model w ith systematic trends reported in a number of studies of the X-ray spectral properties of AGNs (indicating that type 2 objects are harder than type 1 and that the relative amount of the reflecte d radiation is larger in the latter). On the other hand, the model with a rapidly rotating black hole predicts larger apparent luminosities for objects observed at higher θobs, while an opposite property (i.e. type 1 objects being more luminous than type 2) was revealed in the Integral data. We investigate also the hadronic γ-ray emission from hot flows and we find much higher (by orders of magnitude) γ-ray luminosities than estimated in previous studies. If ne arby AGNs contain rapidly rotating black holes and weakly magnetized hot flows, their γ-ray emission should be detectable by current γ-ray detectors.