Soft X-ray transient light curves as standard candles: exponential versus linear decays

A recent paper by King & Ritter proposed that the light curves of soft X-ray transients (SXTs) are dominated by the effect of irradiation of the accretion disc by the central X-rays. This prevents the onset of the cooling wave which would otherwise return the disc to the quiescent state, and so prolongs the outbursts beyond those in dwarf nova discs. KR show that the decay of thr resulting X-ray light curve should be exponential or linear depending on whether or not the observed peak X-ray luminosity is sufficient to ionize the outer edge of the accretion disc. Here we examine the observed X-ray decays, and show that they are exponential or linear according to whether the peak luminosity is greater or smaller than the critical value defined by KR, strongly suggesting that the light curves are indeed irradiation dominated. We show further that the occurrence of an exponential or linear decay tends to favour the same type of decay in subsequent outbursts, so that systems usually show only one or the other type. We use the equations of KR and the observed X-ray light curve to determine the size R-h of the hot disc at the peak of the outburst. For exponential decays, R-h is found to be comparable to the circularization radius, as expected, since the disc consists entirely of material transferred from the secondary since the previous outburst. Further, R-h is directly proportional to the time at which one sees the secondary maximum (t(s)), as expected, if t(s) is the viscous time-scale of the irradiated disc. This implies that the orders of magnitude of the viscosity parameter a and disc aspect ratio H/R are such that alpha(H/R) similar to 0.01. Observation of a secondary maximum calibrates the peak luminosity and gives the distance (D-kpc) to the source as D-kpc = 4 3 x 10(-5) t(s)(3/2)eta(1/2)f(1/2)F(p)(-1/2)tau(d)(-1/2), where F-p is the peak flux, tau(d) is the e-folding time of the decay in days, eta is the radiation efficiency parameter and f is the ratio of the disc mass at the start of the outburst to the maximum possible mass.