Probing the formation of low-mass x-ray binaries in globular clusters and the field

Kundu, Arunav, Maccarone, Thomas J. and Zepf, Stephen E. (2007) Probing the formation of low-mass x-ray binaries in globular clusters and the field. Astrophysical Journal, 662 (1), 525-543. (doi:10.1086/518021).

Abstract

We present an analysis of LMXBs and GCs in five early-type galaxies using Chandra X-ray and HST optical data. Of the 186 LMXBs within the optical fields of view, 71 are in GCs, confirming that LMXBs are formed particularly efficiently in clusters due to dynamical interactions. However, there is no statistically significant correlation between the distance of a cluster from the center of its host galaxy and its LMXB hosting probability. LMXBs are preferentially found in luminous and metal-rich GCs. Metal-rich clusters are 3.4 times more likely to host LMXBs than metal-poor ones. This is slightly higher than that measured in other surveys, likely because of larger contamination of the GC sample in previous ground-based data sets and the inclusion of galaxies with intermediate-age clusters in others. Intriguingly, the LMXBs in NGC 1399 are preferentially in the reddest clusters of the metal-rich GC subsystem. This indicates that the red peak of the bimodal GC color distribution itself encompasses clusters with a range of enrichment histories. The strength of this effect varies from galaxy to galaxy, possibly indicating differences in their metal enrichment histories. Field LMXBs in our program galaxies are more concentrated toward the center of their host galaxies than GC LMXBs. This suggests that a majority of field LMXBs are formed in situ and are not a population that has escaped from current GCs. This is consistent with previous specific frequency-based studies. The brightest X-ray sources in GCs appear to be preferentially associated with luminous, metal-rich clusters. We show that it is probable that some of these clusters host multiple bright LMXBs, while the probability is much lower for metal-poor GCs. If this interpretation is correct, our study implies that LMXBs in more metal-rich cluster systems should reveal a longer high-luminosity X-ray tail and show less X-ray variability than metal-poor cluster populations.

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