The Brightest Cluster X-ray Sources
Abstract
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 sun 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 sun. 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 sun
- Publication:
-
The Astrophysical Journal
- Pub Date:
- May 2011
- DOI:
- 10.1088/2041-8205/732/2/L28
- arXiv:
- arXiv:1104.0751
- Bibcode:
- 2011ApJ...732L..28K
- Keywords:
-
- accretion;
- accretion disks;
- black hole physics;
- globular clusters: general;
- X-rays: binaries;
- X-rays: galaxies;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- ApJ Letters, in press