Effect of X-ray Irradiation on the accretion discs in X-ray binaries.

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 disc. The structure of the outer disc 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 disc, 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 disc which in turn may affect the inner accretion rate. To understand the qualitative behavior of the disc in such a scenario, we adopt simplistic assumptions that the disc is fully ionized and is not warped. We develop and use a time-dependent global hydro-dynamical code to study the effect of a sinusoidal accretion rate perturbation introduced at a specific radius. The response of the disc, especially the inner accretion rate, to such perturbations at different radii and with different time periods is shown. While we did not find any oscillatory or limit cycle behavior, our results show that irradiation enhances the X-ray variability at time-scales corresponding to the viscous time-scales of the irradiated disc.