The Unusual X-ray Binaries of the Globular Cluster Ngc 6652

Abstract

Our 5 ks Chandra ACIS-S observation of the globular cluster NGC 6652 detected seven X-ray sources, three of which were previously unidentified. This cluster hosts a well-known bright low-mass X-ray binary, source A (or XB1832-330). Source B shows unusual rapid flaring variability, with an average L-X(0.5-10 keV) similar to 2 x 10(34) erg s(-1), but with minutes-long flares up to L-X = 9 x 10(34) erg s(-1). Its spectrum can be fit by an absorbed power law of photon index Gamma similar to 1.24 and hardens as the count rate decreases. This suggests that part or all of the variation might be due to obscuration by the rim of a highly inclined accretion disk. Sources C and D, with L-X similar to 10(33) erg s(-1), have soft and unusual spectra. Source C requires a very soft component, with a spectrum peaking at 0.5 keV, which might be the hot polar cap of a magnetically accreting polar cataclysmic variable. Source D shows a soft spectrum (fit by a power law of photon index similar to 2.3) with marginal evidence for an emission line around 1 keV; its nature is unclear. The faint new sources E, F, and G have luminosities of 1-2 x 10(32) erg s(-1), if associated with the cluster (which is likely). E and F have relatively hard spectra (consistent with power laws with photon index similar to 1.5). G lacks soft photons, suggesting absorption with N-H > 10(22) cm(-2).