DO QUIESCENT SOFT-X-RAY TRANSIENTS CONTAIN MILLISECOND RADIO PULSARS

Soft X-ray transients (SXRTs) in outburst show properties similar to t hose of persistent Low-mass X-ray binaries (LMXRBs) and therefore like ly contain an old weakly magnetic neutron star spun up by accretion to rques. We investigate the conditions under which a detectable radio pu lsar signal can be produced by the rapidly rotating neutron star in th e quiescent phase of a SXRT. As the mass inflow rate toward the neutro n star decreases during the decay of an outburst, the radius of the ne utron star magnetosphere might expand beyond the corotation radius, in hibiting accretion onto the neutron star due to the ''centrifugal barr ier.'' Hence, the minimum observed accretion-induced X-ray luminosity at the end of an outburst provides constraints on the neutron star mag netic field, B, and spin period, P. Based on current measurements, SXR Ts can lie in the region of the B-P diagram which is characteristic of recycled millisecond radio pulsars. If this were the case, the radio pulsar emission from a SXRT can resume only when the mass inflow rate from the companion star decreases by a few orders of magnitude below t he ''centrifugal barrier'' threshold. The persistent emission (approxi mately 10(33)-10(34) ergs s-1) detected in the quiescent state of a fe w SXRTs might result from accretion onto the neutron star surface, fro m accretion down to the magnetospheric radius (if the ''centrifugal ba rrier'' is closed), or from the neutron star cooling. In the latter tw o cases, the possibility of one of these SXRTs turning on as a radio p ulsar is far more likely.