Light curves of six bright soft X-ray transients in M31

Abstract

Disc irradiation is thought to be capable of explaining the global behaviour of the light curves of soft X-ray transients (SXTs). Depending on the strength of the central X-ray emission in irradiating the disc, the light curve may exhibit an exponential or a linear decay. The model predicts that in brighter transients a transition from exponential decline to a linear one may be detectable. In this study, having excluded super-soft sources and hard X-ray transients, a sample of bright SXTs in M31 (L-peak > 10(38) erg s(-1)) has been studied. The expected change in the shape of the decay function is only observed in two of the light curves from the six light curves in the sample. Also, a systematic correlation between the shape of the light curve and the X-ray luminosity has not been seen.