Highly compact binary X-ray sources

Abstract

It is proposed that many of the X-ray sources associated with the galactic bulge are accreting neutron stars or black holes with masses of at least about 1 solar mass in ultrashort-period binary systems with very low-mass (no more than about 0.5 solar mass) stellar companions. It is demonstrated that this hypothesis may provide a coherent picture of the evolutionary history and observational properties of these sources, including the apparent lack of conspicuous optical counterparts and the apparent absence of X-ray eclipses.