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

A. Niedzwiecki, F. -G. Xie, A. A. Zdziarski

Published 2010-02-09Version 1

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-thin 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 rapidly 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 flow depends on the inclination angle of the line of sight to the symmetry axis, with higher angles characterised by a harder slope and a higher cut-off energy. 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 with 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 reflected 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 viewing angles, while an opposite property (i.e. type 1 objects being more luminous than type 2) was revealed in the Integral data.