Advection-Dominated Accretion Flows (ADAFs) around Neutron Stars and Black Holes in Transient X-ray Binaries

Kristen Menou (Harvard-Smithsonian CfA, Cambridge, USA & DARC, Observatoire de Paris-Meudon, France)

12:00pm Monday October 5 in the 3E - Theory Conference Room

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 strongly 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 accretion occurs via an ADAF in these systems and if the BHs have event horizons. The luminosities of quiescent NS SXTs are also consistent with the predictions of binary evolution models, but only if an efficient propeller effect prevents most of the mass accreted in the ADAF from reaching the neutron star surface. We develop a model for the propeller effect that accounts for the observed luminosities of quiescent NS SXTs.