Chandra and Hubble Space Telescope Study of the Globular Cluster NGC 288

Abstract

We report on the Chandra X-Ray Observatory observations of the globular cluster NGC 288.We detect four X-ray sources within the core radius and seven additional sources within the half-mass radius down to a limiting luminosity of LX ¼ 7 ; 1030 ergs s 1 (assuming cluster membership) in the 0.3Y7 keV band. We also observed the cluster with the Hubble Space Telescope Advanced Camera for Surveys and identify optical counterparts to seven X-ray sources out of the nine sourceswithin the HST field of view. Based on the X-ray and optical properties, we find 2Y5 candidates of cataclysmic variables (CVs) or chromospherically active binaries and 2Y5 background galaxies inside the half-mass radius. Since the core density of NGC 288 is very low, the number of faint X-ray sources of NGC 288 found in the Chandra and HSTobservations is higher than the prediction on the basis of the collision frequency.We suggest that the CVs and chromospherically active binaries are primordial in origin, in agreement with the theoretical expectation.