LOW-MASS X-RAY BINARY EVOLUTION AND THE ORIGIN OF MILLISECOND PULSARS

We consider the evolution of low-mass X-ray binaries (LMXBs). We show that X-ray irradiation of the companion stars causes these systems to undergo episodes of rapid mass transfer followed by detached phases. The systems are visible as bright X-ray binaries only for a short part of each cycle, so that their space density must be considerably larger than previously estimated. This removes the difficulty in regarding LMXBs as the progenitors of low-mass binary pulsars. We identify the low-accretion-rate phase of the cycle with the soft X-ray transients. We show that 3 hr is likely to be the minimum orbital period for LMXBs with main-sequence companions and suggest that the evolutionary endpoint for many LMXBs may be systems which are the sites of gamma-ray bursts.