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Astron. Astrophys. 358, 910-922 (2000)

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7. Summary and discussion

In the three low-reddened clusters [FORMULA]Cen, NGC 6397 and NGC 6752 we have detected a total of 17 dim X-ray sources, of which 5 are well outside the core. The X-ray luminosities of these sources are listed in Table 8, and plotted in Fig. 8. The interpretation of Fig. 8 must be made with some care. First, sources outside the core may not belong to the cluster; the faintest core source in [FORMULA]Cen may be a fore- or background source. Second, the conversion of observed countrate to luminosity depends on the assumed spectrum, and from PSPC observations we know that different sources have different spectral parameters (Johnston et al. 1994). For example, the 0.6 keV black body spectrum used for the sources in [FORMULA]Cen gives a 40% higher flux for the same countrate than an assumed 0.6 keV bremsstrahlung spectrum would give. The bremsstrahlung spectrum is used for the three other clusters. Third, the detection limits in NGC 6397, NGC 6752 and 47 Tuc are higher in the cores, where the point spread functions of sources overlap, than outside the core. Such a difference is not present in [FORMULA]Cen. Fourth, we show the average luminosity, and several sources are known to be variable.

[FIGURE] Fig. 8. X-ray luminosities of dim sources in four globular clusters. Sources in and outside the core are shown as [FORMULA] and [FORMULA], respectively. Data points are from Table 8, and for 47 Tuc from Verbunt & Hasinger (1998) slightly modified for an assumed 0.6 keV bremsstrahlung spectrum. In all cases the detection limits in and outside the core are close to the lowest detected luminosities in and outside the cores.


Table 8. X-ray luminosities in erg s-1 in the 0.5-2.5 keV band of the dim X-ray sources in globular clusters described in this paper. For sources in [FORMULA]Cen we assume a 0.6 keV black body spectrum; for those in NGC 6397 and NGC 6752 a 0.6 keV bremsstrahlung spectrum. For the same countrate, the blackbody spectrum corresponds to a flux higher by about 40% than the bremsstrahlung spectrum.

With these points in mind, we note from Fig. 8 that in all clusters except possibly [FORMULA]Cen the most luminous sources appear to be in the cluster core. The main difference between [FORMULA]Cen and the other clusters is that the collision frequency in [FORMULA]Cen is so low that one expects no low-mass X-ray binaries in it, and that most cataclysmic variables in it will be evolved from primordial binaries (Verbunt & Meylan 1988, Davies 1997). In addition, the mass segregation in this cluster is very low. Thus in [FORMULA]Cen there is no marked difference between the core and the regions outside the core.

In each cluster we detect sources down to the detection limit; this suggests that more sensitive observations will detect more sources. In the cores of NGC 6397 and NGC 6752 the detection of more source will also require better imaging, so that the faint sources can be detected against the brighter ones. We do not detect a difference between the luminosities of sources in the collapsed globular cluster NGC 6397 and the much less concentrated globular cluster NGC 6752. On the other hand, the highly concentrated cluster 47 Tuc contains three sources which are an order of magnitude brighter than the brightest sources in NGC 6397 and NGC 6752.

Viable optical counterparts have been suggested for only five among the 26 sources shown in Fig. 8, all of them probable cataclysmic variables. We compare the ratio of X-ray flux to optical flux of these sources with the ratios measured for cataclysmic variables and for RS CVn systems in the Galactic Disk in Fig. 9. It is seen that the suggested optical counterparts for the sources in NGC 6397 and NGC 6752 lead to ratios which are compatible with those of cataclysmic variables, whereas those in 47 Tuc are too bright in X-rays, in agreement with Fig. 8. If these sources are indeed cataclysmic variables, their excessive X-ray luminosity needs to be explained; alternatively, the suggested identifications may be chance coincidences (as discussed by Verbunt & Hasinger 1998). All suggested counterparts lead to higher X-ray to optical flux ratios than those of RS CVn binaries.

[FIGURE] Fig. 9. X-ray countrates of the dim sources in globular clusters as a function of their visual magnitude, compared with the ROSAT PSPC countrates and visual magnitudes of various types of cataclysmic variables (data from Verbunt et al. 1997; filled symbols represent systems first discovered in X-rays and only subsequently identified as cataclysmic variables, i.e. X-ray selected systems) and with RS CVn systems (data from Dempsey et al. 1993) respectively. PSPC countrates of the dim cluster sources have been computed for an assumed 0.6 keV bremsstrahlung spectrum, corrected for absorption, from the observed HRI countrates. Visual magnitudes are also corrected for absorption. T indicate sources in 47 Tuc (X 9 and X 19, V as estimated by Verbunt & Hasinger 1998), A in NGC 6397 (X 4b and X 4c, V from Cool et al. 1998), B in NGC 6752 (X 7a, V from Bailyn et al. 1996). The dotted lines indicate a constant ratio of X-ray to optical flux.

The accurate positions that we determine for individual sources are valid for separately detected sources in particular. In the case of overlapping sources, we do not have unique solutions. Thus, in the core of NGC 6397 fits with 5 and 6 sources are both acceptable, at similar quality; and we cannot exclude that more sources contribute to the observed flux, which would invalidate our derived positions.

Binaries may reside away from the core either because the cluster has undergone little mass segregation, or because a three-body interaction (i.e. a close encounter of a binary with a single star) in the core has expelled the binary from the core (e.g. Hut et al. 1992). In the latter case the binary is expected to be eccentric immediately after being expelled; tidal forces may in time circularize the orbit again. Such binaries are only a minority of the overall binary population of a cluster; however, X-ray observations may preferably select such binaries if tidal forces act in them. Since sources away from the core can be fore- or background sources, optical identification of them is required to settle whether they belong to the cluster or not. Our accurate positions should help in finding such counterparts.

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© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000