Hysteresis in the light curves of soft X-ray transients

Using Proportional Counter Array (PCA) data from the Rossi X-Ray Timing Explorer (RXTE), we track the spectral states of the neutron star transient system Aql X-1 through a complete outburst cycle. We find a hard-to-soft state transition during the very early, rising phase of the outburst, and show that there is a hysteresis effect such that the transition back to the hard state occurs at a luminosity similar to5 times lower than the hard-to-soft transition. This hysteresis effect rules out the propeller mechanism as the sole cause of state transitions in Aql X-1. Assuming the propeller mechanism only operates at a luminosity equal to or below that of the observed soft-to-hard transition requires that the magnetic field of Aql X-1 be less than 7 x 10(7) G, the lowest neutron star field known to date. To compare the state transition behaviour of Aql X-1 with that found in transient black hole systems, we use RXTE All-Sky Monitor (ASM) data to compute hardness-intensity diagrams for four black hole candidate transients where the ASM data should also give us state information throughout much of the outburst cycles. In all four systems, we find evidence for a hard-to-soft state transition during the rising outburst phase and for the source staying in a soft state down to much lower luminosities during the declining phase, i.e. a hysteresis effect. This similarity suggests a common origin for state transitions in low magnetic field neutron star and black hole systems, and the hysteresis effect rules out the 'strong ADAF (advection-dominated accretion flow) principle' for determining the state of an accretion disc. We discuss the general implications of these observations for current models of state transitions. We note the contrast to previous observations of the non-transient systems Cygnus X-1 and X-3, which do not show a hysteresis effect.