2011ApJ...732L..28K

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 M₂≃ 0.34 M_☉. The beaming here is tighter (b ∼ (2.5-9)x10^–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_☉