X - Ray Observations of Globular Cluster

A ROSAT HRI X-ray image was obtained of the open globular cluster NGC 288, which is located near the South Galactic Pole. This is the first deep X-ray image of this system. We detect a Low Luminosity Globular Cluster X-ray source (LLGCX) RXJ005245.0−263449 with an X-ray luminosity of (5.5± 1.4)× 1032 ergs s−1 (0.1–2.0 keV), which is located very close to the cluster center. There is evidence for X-ray variability on a time scale of ∼<1 day. The presence of this LLGCX in such an open cluster suggests that dense stellar systems with high interaction rates are not needed to form LLGCXs. We also searched for diffuse X-ray emission from NGC 288. Upper limits on the X-ray luminosities are LX < 9.5×10 32 ergs s−1 (0.52–2.02 keV) and LX < 9.3×10 32 ergs s−1 (0.11–0.41 keV). These imply upper limits to the diffuse X-ray to optical light ratios in NGC 288 which are lower than the values observed for X-ray faint early-type galaxies. This indicates that the soft X-ray emission in these galaxies is due either to a component which is not present in globular clusters (e.g., interstellar gas, or a stellar component which is not found in low metallicity Population II systems), or to a relatively small number of bright Low Mass X-ray Binaries (LMXBs). Subject headings: globular clusters: general — globular clusters: individual (NGC 288) — galaxies: elliptical and lenticular, cD — ultraviolet: stars — X-rays: general — X-rays: stars