Chandra observation of the globular cluster NGC 6440 and the nature of cluster X-ray luminosity functions

As part of our campaign to determine the nature of the various source populations of the low-luminosity globular cluster X-ray sources, we have obtained a Chandra X-Ray Observatory ACIS-S3 image of the globular cluster NGC 6440, We detect 24 sources to a limiting luminosity of ∼2 × 10 ³¹ ergs s^-1 (0.5-2.5 keV) inside the cluster's half-mass radius, all of which lie within ∼2 core radii of the cluster center. We also find excess emission in and around the core that could be due to unresolved point sources. Based on X-ray luminosities and colors, we conclude that there are 4-5 likely quiescent low-mass X-ray binaries and that most of the other sources are cataclysmic variables. We compare these results to Chandra results from other globular clusters and find the X-ray luminosity functions differ among the clusters.