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

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 NGC362 (a double source), NGC6121 (marginally significant), NGC6139, and NGC6266; and outside the cores of NGC6205, NGC6352 and NGC6388. More accurate positions are determined for the X-ray sources in some ten clusters. The improved position for the source in NGC6341 excludes the suggested ultraviolet counterpart. It is shown that one of the two sources reported near the core NGC6626 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 NGC6304 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 =~0.9keV; 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.Comment: 24 pages, 8 figures, 5 tables, accepted for publication in Astronomy and Astrophysic

Ground and space-based study of two globular cluster CVs: M22 CV1 and M5 V101

As a class of compact binaries with large binding energy, cataclysmic variables formed through close encounters play an important role in the dynamical evolution of globular clusters. As part of a systematic search for CVs undergoing dwarf nova eruptions in globular clusters, our 2004 monitoring programme of M22 detected an outburst of the dwarf nova candidate CV1 during May. We implement the ISIS image subtraction routine to obtain a light curve for an outburst of CV1. We present the outburst light curve as well as HST/WFPC2 photometry in the V, U and near ultra-violet (nUV) bands and a Chandra/ACIS-S spectrum of the object. Our results confirm the DN nature of the outburst and the CV status of the object. We also present the results of a ground-based study of another globular cluster CV, M5 V101 - including quiescent medium-resolution WHT/ISIS spectroscopy in the B and R bands, displaying prominent Balmer and HeI emission, and R-band photometry.Comment: 12 pages, 11 figures. Accepted for publication in MNRA