Transient X-ray binaries in elliptical galaxies

Chandra observations of elliptical galaxies have revealed large numbers of low-mass X-ray binaries (LMXBs) accreting at rates greater than 10(-9) M. yr(-1). One scenario that generates this from an old stellar population is nuclear-driven mass transfer onto a neutron star or black hole from a Roche lobe-filling red giant. However, in our Galaxy, most of these systems accrete sporadically as transients because of a thermal instability in the accretion disk. Using the common criterion for disk instability (including irradiation), we find that this mode of mass transfer leads to transient accretion for at least 75% of the binary's life. Repeated Chandra observations of elliptical galaxies should reveal this population. The recurrence times might be very long (similar to100-10,000 yr depending on the orbital period at the onset of mass transfer), and outbursts might last for 1-100 yr. Mass transferring binaries can also be formed in old populations via interactions in dense stellar environments, such as globular clusters. These tend to have shorter orbital periods and are more likely stable accretors, making them apparently a large fraction of an elliptical galaxy's LMXB population.