The Optical Properties of Globular Clusters Containing Low-Mass X-ray Binaries

Details The Optical Properties of Globular Clusters Containing Low-Mass X-ray Binaries The Optical Properties of Globular Clusters Containing Low-Mass X-ray Binaries

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006head....9.0161s&link_type=abstract

American Astronomical Society, HEAD meeting #9, #1.61; Bulletin of the American Astronomical Society, Vol. 38, p.339

Astronomy and Astrophysics

Astronomy

Scientific paper

Low-mass X-ray binaries (LMXBs) form efficiently in globular clusters (GCs). By combining Chandra and Hubble Advanced Camera for Surveys observations of early-type galaxies, the most accurate identification of LMXBs and GCs to date allows us to probe the LMXB-GC connection. We explore the optical properties of 270 GCs with LMXBs and 6,488 GCs without detectable X-ray emission from a sample of eleven massive early-type galaxies in the Virgo cluster. Globular clusters that are more massive, are redder, and have smaller radii are more likely to contain LMXBs. By fitting the dependence of the expected number of LMXBs per GC λt on the GC mass, color, and half-mass radius
λt α M1.24±0.08 10(0.9+0.2/-0.1)(G475-Z850) rh-2.2+0.3/-0.4,
we rule out that the number of LMXBs per GC is linearly proportional to GC mass and predict that most GCs with high X-ray luminosities contain a single LMXB. The detailed dependence of λt on GC properties appears mainly due to a dependence on a combination of mass and radius, and a dependence on color, that are essentially equivalent to a dependence on the binary encounter rate Γh and the metallicity Ζ,
λt α Γh0.82±0.05 Ζ0.39±0.07.
We believe our analysis is the most direct evidence to date that indicates dynamical formation and metallicity play the primary roles in forming LMXBs in GCs. The encounter rate dependence is consistent with X-ray sources and radio pulsars in Galactic-GCs. Binary hardening can explain this shallower than linear dependence for all three source types; however, we note that our use of Γh as a proxy for the encounter rate, particularly if core-collapsed extragalactic GCs preferentially contain LMXBs, needs further testing in nearer galaxies. The metallicity dependence is most consistent with either a metallicity dependent variation in the number of neutron stars and black holes per GC or effects from irradiation-induced winds.

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