Black Hole and Neutron Star Transients in Quiescence

We consider the X-ray luminosity difference between neutron star and blackhole soft X-ray transients (NS and BH SXTs) in quiescence. The currentobservational data suggest that BH SXTs are significantly fainter than NS SXTs.The luminosities of quiescent BH SXTs are consistent with the predictions ofbinary evolution models for the mass transfer rate if (1) accretion occurs viaan ADAF in these systems and (2) the accreting compact objects have eventhorizons. The luminosities of quiescent NS SXTs are not consistent with thepredictions of ADAF models when combined with binary evolution models, unlessmost of the mass accreted in the ADAF is prevented from reaching the neutronstar surface. We consider the possibility that mass accretion is reduced inquiescent NS SXTs because of an efficient propeller and develop a model of thepropeller effect that accounts for the observed luminosities. We argue thatmodest winds from ADAFs are consistent with the observations while strong windsare probably not.Comment: LateX, 37 pages, 7 figures; Accepted for publication in The Astrophysical Journa