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

Abstract

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 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.

Optical Identification of Multiple Faint X‐Ray Sources in the Globular Cluster NGC 6752: Evidence for Numerous Cataclysmic Variables1

“…We note that if this is the case, then NGC 6752 becomes the second globular cluster for which the optical determination of its center has been corrected by an X-ray observation (the first one being NGC 6541; see Fig. 1 of Verbunt 2001).…”

“…The radio pulsar in M28 is another securely identified faint X-ray source (Lyne et al 1987;Saito et al 1997). Some identifications, like those in M13, remain in doubt (Verbunt 2001).…”

“…Whereas virtually all of the core sources are related to the globular clusters, some of the sources outside the cores may be in the background or foreground (Verbunt 2001 and references therein). Because of crowding and the limited accuracy of even the ROSAT positions (varying from 2 00 to 5 00 , depending on whether a secure optical identification allows accurate determination of the bore sight correction), optical identifications of faint X-ray sources in globular cluster cores remained tentative before the Chandra era, and some suggestions have been disproved with more accurate X-ray positions, as in 47 Tuc (Verbunt & Hasinger 1998) and M92 (Geffert 1998;Verbunt 2001). The only secure optical identification was that of a dwarf nova with a faint X-ray source 12 core radii from the center of NGC 5904 (Hakala et al 1997;Margon, Downes, & Gunn 1981).…”

ChandraObservation of the Globular Cluster NGC 6440 and the Nature of Cluster X‐Ray Luminosity Functions

“…The Chandra observations have resolved several ROSAT sources into multiple sources and have also detected sources with lower luminosities. As a result, the number of faint sources is already larger in the few clusters observed with Chandra than in the complete ROSAT data set described by Verbunt (2001). We plot the cumulative luminosity functions of four clusters observed with Chandra in Figure 3.…”

An XMM-Newton observation of the globular cluster Omega Centauri

“…Binaries such as CVs, low mass X-ray binaries either with a neutron star or a black hole, or active X-ray binaries (RS CVn or BY Dra systems) or millisecond pulsars could thus form. Some of these binaries have already been found as faint X-ray sources in ω Cen (Verbunt 2001).…”