Low‐Mass X‐Ray Binaries in Six Elliptical Galaxies: Connection to Globular Clusters

Abstract

We present a systematic study of the low-mass X-ray binary (LMXB) populations of six elliptical galaxies, aimed at investigating the detected LMXB-globular cluster (GC) connection. We utilize Chandra archival data to identify X-ray point sources and HST archival data supplemented by ground observations to identify 6173 GCs. After screening and cross-matching, we associate 209 LMXBs with red GC (RGCs) and 76 LMXBs with blue GCs (BGCs), while we find no optical GC counterpart for 258 LMXBs. This is the largest GC-LMXB sample studied so far. We confirm previous reports suggesting that the fraction of GCs associated with LMXBs is ~3 times larger in RGCs than in BGCs, indicating that metallicity is a primary factor in the GC LMXB formation. We find that GCs located near the galaxy center have a higher probability of harboring LMXBs than those in the outskirts, suggesting that there must be another parameter (in addition to metallicity) governing LMXB formation in GCs. This second parameter, dependent on the galactocentric distance, may be a distance dependent encounter rate. We find no significant differences in the shape of X-ray luminosity function, LX/LV distribution, X-ray spectra among RGC, BGC, and field LMXBs. The similarity of the X-ray spectra is inconsistent with the irradiation-induced stellar wind model prediction. The similarity of the X-ray luminosity functions (XLFs) of GC LMXBs and field LMXBs indicates that there is no significant difference in the fraction of black hole binaries present. We cannot either prove or reject the hypothesis that all LMXBs were formed in GCs.