4.5. Comparing clusters

In comparing the different clusters, it must be taken into account to which limit sources can be detected. Low-luminosity low-mass X-ray binaries with a neutron star tend to be more luminous than cataclysmic variables, which in turn tend to be more luminous than magnetically active binaries. This ordering is reflected in the numbers of currently known cataclysmic variables and magnetically active binaries listed in Table 4 as a function of the detection limit.

Another number that is important is the estimated number of close encounters between stars in the globular cluster. Pooley et al. (2003) show that the number of X-ray sources detected in a globular cluster above an observational threshold of L_x 4 × 10³⁰ erg s^-1(0.5-6 keV) scales quite well with this number, as shown in Figure 13. Heinke et al. (2003d) find that the number of cataclysmic variables alone (at L_x × 10³¹ erg s^-1) possibly increases slower with central density than predicted by proportionality to the number of close encounters.

Figure 13. Number N of X-ray sources with Lx 4 × 1030 erg s-1(0.5-6 keV) detected in globular clusters, as a function of the collision number . is a measure of the number of close encounters between stars in a cluster (see Eqs. 5, 6). The luminosity limit implies that most sources are cataclysmic variables. In general N scales quite well with , indicating that cataclysmic variables in globular clusters are formed via close encounters between a white dwarf and another star or a binary. Arrows indicate lower limits. NGC6397 doesn't follow the general trend. From Pooley et al. (2003).

An exception to this scaling is NGC6397. This cluster has a higher number of neutron star binaries and cataclysmic variables than expected on the basis of its rather low collision number. Remarkably, the number of magnetically active binaries in this cluster is not very high, and this is reflected in a relatively flat X-ray luminosity function (Pooley et al. 2002b). If it is true, as argued by Pooley et al. (2003), that the high number of neutron star binaries and cataclysmic variables in NGC6397 is due to its being shocked and stripped in multiple passages near the galactic centre, it has to be explained why these mechanisms are more efficient in removing magnetically active binaries than in removing cataclysmic variables and binaries with neutron stars.