Do quiescent soft X-ray transients contain millisecond radio pulsars?

Abstract

Soft X-ray transients (SXRTs) in outburst show properties similar to those of persistent Low-mass X-ray binaries (LMXRBs) and therefore likely contain an old weakly magnetic neutron star spun up by accretion torques. We investigate the conditions under which a detectable radio pulsar signal can be produced by the rapidly rotating neutron star in the quiescent phase of a SXRT. As the mass inflow rate toward the neutron star decreases during the decay of an outburst, the radius of the neutron star magnetosphere might expand beyond the corotation radius, inhibiting accretion onto the neutron star due to the 'centrifugal barrier.' Hence, the minimum observed accretion-induced X-ray luminosity at the end of an outburst provides constraints on the neutron star magnetic field, B, and spin period, P. Based on current measurements, SXRTs 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 the 'centrifugal barrier' threshold. The persistent emission (approximately 10<SUP>33</SUP> to 10<SUP>34</SUP> ergs/sec) detected in the quiescent state of a few SXRTs might result from accretion onto the neutron star surface, from accretion down to the magnetospheric radius (if the 'centrifugal barrier' is closed), or from the neutron star cooling. In the latter two cases, the possibility of one of these SXRTs turning on as a radio pulsar is far more likely.