ASCA observations of transient x-ray sources in quiescence

Soft X-ray transients are most probably low-mass binary systems containing either a neutron star (NS-LMXB) or a black hole (BH-LMXB). We have systematically investigated thirteen soft X-ray transients (four NS-LMXBs and nine BH-LMXBs) in the quiescent state using the available ASCA data. The luminosities of the observed NS-LMXBs in quiescence are on the order of similar to 10(32-33) erg s(-1), whereas those of the BH-LMXBs are found to be systematically lower, most of them giving upper limits in the range below 10(32) erg s(-1). The quiescent luminosity of LMXBs may be explained in terms of an optically thin, advection-dominated accretion disk, for which most of the thermal and kinetic energies of accreting matter are advected into the compact object with little X-ray emission. However, this scenario predicts a much larger luminosity for NS-LMXBs than observed, if the advected energies are liberated at the neutron-star surface, which is absent in BH-LMXBs. Mass accretion onto the neutron-star surface may be suppressed through some mechanism, such as the so-called propeller effect. The soft component observed in the NS-LMXBs is most probably emission from the magnetic pole regions of the neutron star.