Mass transfer during low-mass X-ray transient decays

The outbursts of low-mass X-ray binaries are prolonged relative to those of dwarf nova cataclysmic variables as a consequence of X-ray irradiation of the disc. We show that the time-scale of the decay light curve and its luminosity at a characteristic time are linked to the radius of the accretion disc. Hence, a good X-ray light curve permits two independent estimates of the disc radius. We apply this to a number of sources: 4U 1543-475, SAX J1808.4-3658, XTE J1751-305, XTE J0929-314, XTE J1807-294, GRO J1744-28, GX339-4, XTE J1550-564, GRO J1655-40 and 4U 1705-44. In the case of the millisecond pulsars SAX J1808.4-3658 and XTE J0929-314, the agreement between these estimates is very strong. Our analysis allows new determinations of distances and accretion disc radii. Our analysis will allow determination of accretion disc radii for sources in external galaxies, and hence constrain system parameters where other observational techniques are not possible. We also use the X-ray light curves to estimate the mass transfer rate. The broken exponential decay observed in the 2002 outburst of SAX J1808.4-3658 may be caused by the changing self-shadowing of the disc.