Ultracompact Binaries as Bright X-Ray Sources in Elliptical Galaxies

Abstract

Chandra observations of distant (D ~ 10 Mpc) elliptical galaxies have revealed large numbers of low-mass X-ray binaries (LMXBs) accreting at > 10-8 M☉ yr-1. The majority of these LMXBs reside in globular clusters (GCs), and it has been suggested that many of the field LMXBs also originated in GCs. We show here that ultracompact (UC) binaries with orbital periods of 8-10 minutes and He or C/O donors of 0.06-0.08 M☉ naturally provide the observed from gravitational radiation losses alone. Such systems are predicted to be formed in the dense GC environment, a hypothesis supported by the 11.4 minute binary 4U 1820-30, the brightest persistent LMXB in a Galactic GC. These binaries have short enough lifetimes ( 1038 ergs s-1) that we calculate their luminosity function under a steady state approximation. This yields a luminosity function slope in agreement with that observed for 6 × 1037 ergs s-1 < L < 5 × 1038 ergs s-1, encouraging us to use the observed numbers of LMXBs per GC mass to calculate the accumulated number of UC binaries. For a constant birthrate over 8 Gyr, the number of UC binaries that have evolved through this bright phase is ~4000 in a 107 M☉ GC, consistent with dynamical interaction calculations. Perhaps most importantly, if all UC binaries become millisecond radio pulsars, then the observed normalization agrees with the inferred number of millisecond radio pulsars in 47 Tuc and Galactic GCs in general.