The effect of X-ray irradiation on the time-dependent behaviour of accretion discs with stochastic perturbations

Abstract

The UV emission from X-ray binaries, is more likely to be produced by reprocessing of X-rays by the outer regions of an accretion disk. The structure of the outer disk may be altered due to the presence of X-ray irradiation and we discuss the physical regimes where this may occur and point out certain X-ray binaries where this effect may be important. The long term X-ray variability of these sources is believed to be due to stochastic fluctuations in the outer disk, which propagate inwards giving rise to accretion rate variation in the X-ray producing inner regions. The X-ray variability will induce structural variations in the outer disk which in turn may affect the inner accretion rate. To understand the qualitative behaviour of the disk in such a scenario, we adopt simplistic assumptions that the disk is fully ionised and is not warped. We develop and use a time dependent global hydrodynamical code to study the effect of a sinusoidal accretion rate perturbation introduced at a specific radius. The response of the disk, especially the inner accretion rate, to such perturbations at different radii and with different time periods is shown. While we didn't find any oscillatory or limit cycle behaviour, our results show that irradiation enhances the X-ray variability at time-scales corresponding to the viscous time-scales of the irradiated disk.

Modeling the response of a standard accretion disc to stochastic viscous fluctuations

“…The partial differential Equation ( 6) is solved numerically to obtain the time dependent behaviour of an accretion disc using the standard finite differencing scheme with the courant condition imposed as described in Maqbool et al (2015). At an outer boundary radius, the disc values are fixed to the steady state ones while for the inner one we maintain the viscous stress free condition.…”

“…Viscous variations can be then represented by variation in the α parameter. Such simulations have been undertaken for, e.g., to study the effect of these variations on unstable radiation pressure dominated discs (Janiuk & Misra, 2012) and to understand the effect of X-ray irradiation on the time dependent properties of the outer regions of an accretion disc (Maqbool et al, 2015). Since the nature especially the time-dependent behaviour of the turbulent viscosity is largely unknown, it is prudent to test different mechanisms by which the viscosity may fluctuate.…”