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 \sim 0.2 { M _ { \odot } } , and orbital periods below 5 minutes .
Accretion is slightly super–Eddington , and makes these systems ultraluminous sources ( ULXs ) with rather mild beaming factors b \sim 0.3 .
Their theoretical luminosity function flattens slightly just above { L _ { Edd } } and then steepens at \sim 3 { L _ { Edd } } .
It predicts of order 2 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 } \mathrel { \raise 1.29 pt \hbox { $ \sim$ } \mkern - 14.0 mu \lower 1.72 pt \hbox { $ - $ } } 0 % .34 { M _ { \odot } } .
The beaming here is tighter ( b \sim 2.5 - 9 \times 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 \sim 1000 { M _ { \odot } }