THE BRIGHTEST CLUSTER X-RAY SOURCES

There have been several recent claims of black hole binaries in globular clusters. I show that these candidate systems could instead be ultracompact X-ray binaries (UCXBs) in which a neutron star accretes from a white dwarf. They would represent a slightly earlier evolutionary stage of known globular cluster UCXBs such as 4U 1820-30, with white dwarf masses ~0.2 M ? and orbital periods below 5 minutes. Accretion is slightly super-Eddington and makes these systems ultraluminous sources with rather mild beaming factors b ~ 0.3. Their theoretical luminosity function flattens slightly just above L Edd and then steepens at ~3L Edd. It predicts of order two detections in elliptical galaxies such as NGC 4472, as observed. The very bright X-ray source HLX-1 lies off the plane of its host S0a galaxy. If this is an indication of globular cluster membership, it could conceivably be a more extreme example of a UCXB with white dwarf mass M 2 0.34 M ?. The beaming here is tighter (b ~ (2.5-9) ? 10?3), but the system's distance of 95?Mpc easily eliminates any need to invoke improbable alignment of the beam for detection. If its position instead indicates membership of a satellite dwarf galaxy, HLX-1 could have a much higher accretor mass ~1000 M ?