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As already mentioned, a limited number of low-luminosity sources has been detected with Chandra in several clusters which contain a luminous X-ray source. The presence of such a source limits the sensitivity with which low-luminosity sources can be detected, because of the wings of the point spread function. The sharp (< 1") images and high sensitivity of the Chandra observations is best used in clusters which do not contain a luminous source. Such observations show that the central regions of several globular clusters contain dozens of sources. As a typical example, the distribution of the sources in NGC6440 is concentrated towards the cluster center; while it spreads beyond the core radius, it is fully contained within the half-mass radius (Figure 8). From this spatial distribution alone, it can be safely asserted that almost all sources detected are related to the globular cluster. In clusters with large core radii and/or half-mass radii, a larger fraction of the detected sources may be fore- or background sources; an example is omegaCentauri.

Figure 8a
Figure 8b

Figure 8. Top: The impact of Chandra on the study of low-luminosity X-ray sources in globular clusters is well illustrated by the observations of 47 Tuc. The grey scale of the smoothed ROSAT-HRI countrate indicates the resolution obtained with this instrument (Verbunt & Hasinger 1998). The circle indicates the position (1sigma region) of the single Einstein source (Hertz & Grindlay 1983), squares indicate the positions of the five ROSAT sources in this region, filled circles are 39 Chandra positions (Grindlay et al. 2001a). Bottom: Projected distribution of X-ray sources in the globular cluster NGC6440. The dashed and solid lines indicate the core and half-mass radii, respectively. From Pooley et al. (2002b). In the case of 47Tuc, each ROSAT source corresponds to one Chandra source; in the case of NGC6440, two sources previously found by ROSAT are both resolved into multiple sources.

In trying to determine the nature of all these X-ray sources, we may be guided by our knowledge from previous satellites, in particular ROSAT. Such guidance allows us to make a preliminary classification of a source based on its X-ray flux and spectrum. If a secure optical counterpart is found - which thanks to the accurate source positions delivered by Chandra is often the case whenever sufficiently deep HST observations are available - the classification of a source can be further based on its optical spectrum, and on the ratio of the X-ray and optical fluxes. A secure classification can also be found if the position of a radio pulsar coincides with that of an X-ray source: radio and X-ray positions are so accurate that the probability of a chance coincidence is virtually negligible for these rare objects.

Our discussion of the low-luminosity sources proceeds through the various classes illustrated in Figure 1, viz. low-luminosity low-mass X-ray binaries, recycled radio pulsars, cataclysmic variables, and magnetically active close binaries. An overview of published Chandra observations of low-luminosity sources in globular clusters is given in Table 4.

Table 4. Published Chandra and XMM observations of low-luminosity X-ray sources in globular clusters. For each cluster we give the lowest detectable luminosity (erg s-1, estimated for the range 0.5-2.5 keV), and the estimated numbers of X-ray sources corresponding to luminous low-mass X-ray binaries (BX) low-luminosity low-mass X-ray binaries (FX), cataclysmic variables (CV), recycled pulsars (PSX), (for comparison: we list the number of radio pulsars known in this cluster from radio PSR), and magnetically active binaries (BY). ->1? (1? <-) indicates that one of the sources in the previous (next) column actually may belong in this column. The final column gives the total number of detected X-ray sources associated with the cluster.

cluster  ref Llow BX FX  CV  PSX (PSR) BY Ntot  

NGC6440   [176] 2 × 1031 1 3       (1)      
NGC6652   [89] 8 × 1032 1 3   ->1?           
Terzan1   [238] 3 × 1033 1 1   ->1?           
Terzan5   [90] 5 × 1032 1 4  5    (4)      
Liller1   [105] ~ 1034 1 3              
47Tuc   [78] 1030 0 2  >30  15 (22) 26 104  
omegaCen   [62] 1031 0 1  >20    (0) 4 ~ 100  
NGC6093   [92] 7 × 1030 0 2  ~ 15        19  
NGC6121   [13] 1 × 1029 0 0  3  1 (1) 14 ~ 20  
NGC6205   [63] 2 × 1031 0 1  4  0 (5)   5  
NGC6397   [79] 3 × 1029 0 1  9  1 (1) 3 ~ 20  
NGC6626   [14] 2 × 1030 0 1  ~ 25  1 (1)      
NGC6656   [232]   0 1?<-  3      ~ 3    
NGC6752   [175] 2 × 1030 0 0  10  1 (5) 3 17  

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