A census with ROSAT of low-luminosity X-ray sources in globular clusters

Astronomical Institute, PO Box 80000, 3508 TA Utrecht, The Netherlands

Corresponding author: F. Verbunt, verbunt@phys.uu.nl

Received: 20 September 2000
Accepted: 4 December 2000

Abstract

I analyze 101 observations from the ROSAT archive to search for X-ray sources in or near 55 globular clusters. New sources are found in the cores of NGC 362 (a double source), NGC 6121 (marginally significant), NGC 6139, and NGC 6266; and outside the cores of NGC 6205, NGC 6352 and NGC 6388. More accurate positions are determined for the X-ray sources in some ten clusters. The improved position for the source in NGC 6341 excludes the suggested ultraviolet counterpart. It is shown that one of the two sources reported near the core of NGC 6626 is spurious, as is the detection of a pulsar period in the PSPC data of this cluster; the central source is resolved in three sources. One source reported previously in NGC 6304 is demoted to an upper limit. For 20 cluster cores better upper limits to the X-ray luminosity are obtained. From a statistical analysis I argue that several sources outside the cluster cores may well belong to the clusters. All spectral energy distributions observed so far are relatively soft, with bremsstrahlung temperatures 0.9 keV; there is evidence however that bremsstrahlung spectra do not correctly describe the spectra. The X-ray luminosity per unit mass for the cluster as a whole does not depend on the concentration; the luminosity per unit mass for the core may increase with the cluster concentration.