AAS 204th Meeting, June 2004
Session 49 X-ray Binary and ULX Populations in Nearby Galaxies
Topical Session, Wednesday, June 2, 2004, 8:30-10:00am, 10:45am-12:30pm, 2:30-4:00pm, 4:15-6:00pm, 601

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[49.13] Ultracompact Binaries as Bright X-Ray Sources in Elliptical Galaxies

L. Bildsten (Kavli Institute for Theoretical Physics, UCSB), C. Deloye (Department of Physics, UCSB)

Chandra observations of distant elliptical galaxies have revealed large numbers of Low Mass X-ray Binaries (LMXBs) accreting near the Eddington limit. 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 binaries with orbital periods of 8-10 minutes and He or C/O donors of 0.06-0.08 solar masses naturally provide the observed accretion rates 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 (less than 3 Myrs) while bright that we calculate their luminosity function under a steady-state approximation. This yields a luminosity function slope in remarkable agreement with that observed. This agreement encourages us to use the observed numbers of LMXBs per GC mass to calculate the accumulated number of ultracompact binaries. For a constant birthrate over 8 Gyrs, the number of ultracompact binaries which have evolved through this bright phase is 4000 in a 10 million solar mass GC, consistent with the dynamical interaction calculations to date. Perhaps most importantly, if all ultracompacts become millisecond radio pulsars, then the observed normalization agrees with the inferred number of millisecond radio pulsars in 47 Tuc.

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