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 ∼2×1031 erg 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 which could be due to unresolved point sources. Based upon 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. Subject headings: globular clusters: individual (NGC 6440) — stars: neutron — cataclysmic variables — binaries: close