Anisotropic X-ray emission in active galactic nucleus accretion discs

Sergei Nayakshin

doi:10.1111/j.1745-3933.2007.00279.x

Abstract

Abstract Straightforward models of X-ray reflection in the inner region of accretion discs predict that the primary X-ray flux and the flux reflected off the surface of the disc should vary together, albeit with a short light traveltime delay. Most of the observations, however, show that the X-ray flux can vary while the reflected features remain constant. Here we propose a simple explanation for this. In all likelihood, the emission of a moderately optically thick magnetic flare atop an accretion disc is anisotropic. A constant energy release rate in a flare will appear to produce a variable X-ray flux as the flare rotates with the accretion disc anchoring the magnetic tube. The reflector, on the other hand, receives a constant X-ray flux from the flare. Since the reflected emission is azimuthally symmetric, the observer will see a roughly constant reflected flux (neglecting relativistic effects). The model does not produce quasi-periodic oscillations if magnetic flux tubes are sheared out faster than they complete one orbit.

Highlights

Rapid X-ray variability of accreting black holes suggests that in many of these sources X-rays are emitted very close to the last stable orbit of an accretion disc presumed to power the emission
We made a very simple point in this paper, emphasising the potential importance of anisotropy of X-ray emitters in accreting black holes. One prediction of this model is an uncorrelated X-ray variability of the continuum and the reflected features on time scales shorter than a dynamical time (R3/2/(GM )1/2, where R is the radius at the location of the flare, and M is the black hole mass), as observed in a number of cases
We do not expect the model to be applicable to variability on time scales longer than the dynamical time

Summary

Introduction

Rapid X-ray variability of accreting black holes (e.g., see the case for AGN in Done & Fabian, 1989) suggests that in many of these sources X-rays are emitted very close to the last stable orbit of an accretion disc presumed to power the emission. It is our view that time-averaged X-ray observations of AGN, with and without broad Fe Kα lines, can all be explained in terms of photo-ionised X-ray reflection models (Nayakshin, 2000).

Results

Conclusion