Ultracompact Binaries as Bright X-Ray Sources in Elliptical Galaxies

Chandra observations of distant elliptical galaxies have revealed largenumbers of Low Mass X-ray Binaries (LMXBs) accreting at rates in excess of10^{-8} solar masses per year. The majority of these LMXBs reside in globularclusters (GCs) and it has been suggested that many of the field LMXBs alsooriginated in GCs. We show here that ultracompact binaries with orbital periodsof 8-10 minutes and He or C/O donors of 0.06-0.08 solar masses naturallyprovide the observed accretion rates from gravitational radiation losses alone.Such systems are predicted to be formed in the dense GC environment, ahypothesis supported by the 11.4 minute binary 4U 1820-30, the brightestpersistent LMXB in a Galactic GC. These binaries have short enough lifetimes(less then 3 Myr) while bright that we calculate their luminosity functionunder a steady-state approximation. This yields a luminosity function slope inagreement with that observed for luminosities in the range of 6E37 ergs/sec to5E38 ergs/sec, encouraging us to use the observed numbers of LMXBs per GC massto calculate the accumulated number of ultracompact binaries. For a constantbirthrate over 8 Gyrs, the number of ultracompact binaries which have evolvedthrough this bright phase is nearly 4000 in a 10 million solar mass GC,consistent with dynamical interaction calculations. Perhaps most importantly,if all ultracompacts become millisecond radio pulsars, then the observednormalization agrees with the inferred number of millisecond radio pulsars in47 Tuc and Galactic GCs in general.Comment: to Appear in Astrophysical Journal Letter