X-ray evidence for white dwarf binaries in globular clusters

An apparently distinct class of low-luminosity (L/sub x/approx.1032--1034 ergs -1) X-ray sources has been discovered in globular clusters from a survey with the IPC detector on the Einstein X-Ray Observatory. The sources appear to be pointlike, and several lie well outside of the cluster core, indicating that their masses are < or approx. =1.0 M/sub sun/. These characteristics, together with previous studies of X-ray sources in globular clusters, make it likely that these sources are caused by accretion onto a white dwarf in a compact binary system. The luminosity function for these sources is consistent with that of galactic plane cataclysmic variables. An extrapolation of the X-ray luminosity function to L/sub x/< or approx. =1031 ergs s-1 indicates that a typical globular cluster contains approx.10/sup 1.3/ white dwarf binaries. Assuming that these binaries form via the tidal capture (two-body) mechanism, the number of white dwarfs in a typical cluster must be approx.10/sup 4.1/; this is consistent with globular cluster mass functions