Low Mass X-ray Binaries and Globular Clusters in Early-Type Galaxies

A high fraction of the Low Mass X-ray Binaries (LMXBs) in early-type galaxies are associated with globular clusters (GCs). Here, we discuss the correlations between LMXBs and GCs in a sample of four early-type galaxies with X-ray source lists determined from Chandra observations. There is some evidence that the fraction of LMXBs associated with globular clusters (fX−GC) increases along the Hubble sequence from spiral bulges (or spheroids) to S0s to Es to cDs. On the other hand, the fraction of globular clusters which contain X-ray sources appears to be roughly constant at fGC−X ∼ 4%. There is a strong tendency for the X-ray sources to be associated with the optically more luminous GCs. However, this correlation is consistent with a constant probability of finding a LMXB per unit optical luminosity; that is, it seems to result primarily from the larger number of stars in optically luminous GCs. The probability of finding a bright LMXB per unit optical luminosity in the GCs is about 1.5× 10−7 LMXBs per L⊙,I for LX & 1 × 10 38 erg s−1 (0.3–10 keV) and rises to about 2.0 × 10−7 LMXBs per L⊙,I at lower X-ray luminosities, LX & 3× 10 37 erg s−1. This frequency appears to be roughly constant for different galaxies, including the bulges of the Milky Way and M31. There is a tendency for the X-ray sources to be found preferentially in redder GCs, which is independent of optical luminosity correlation. This Department of Astronomy, University of Virginia, P. O. Box 3818, Charlottesville, VA 22903-0818; [email protected], [email protected], [email protected], [email protected], Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824; [email protected] Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1090; [email protected]