Black Hole and Neutron Star Transients in Quiescence

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  • 20 October 1998
Abstract
We consider the X-ray luminosity difference between neutron star and black hole soft X-ray transients (NS and BH SXTs) in quiescence. We show that the current observational data clearly suggest that BH SXTs are significantly fainter than NS SXTs. The luminosities of quiescent BH SXTs are consistent with the predictions of binary evolution models for the mass transfer rate if (1) accretion occurs via an ADAF in these systems and (2) the accreting compact objects have event horizons. The luminosities of quiescent NS SXTs are not consistent with the predictions of binary evolution models, unless most of the mass accreted in the ADAF is prevented from reaching the neutron star surface. We propose that the discrepancy is because of an efficient propeller in quiescent NS SXTs and develop a model of the propeller effect that accounts for the observed luminosities. We argue that modest winds from ADAFs are consistent with the observations while strong winds are probably not.

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