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

TL;DR

This study models X-ray emission from hot accretion flows around supermassive black holes, incorporating general relativity to explain spectral differences in AGNs based on black hole spin and viewing angle.

Contribution

It combines GR hydrodynamics with Comptonization to refine predictions of X-ray spectra from accretion flows near rotating black holes, linking spectral properties to AGN unification models.

Findings

Emission is dominated by regions near the black hole's event horizon.

Spectral hardness and cutoff energy depend on the inclination angle.

Predicted luminosity trends differ from some observational data.

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-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.