X-ray sources in omega Centauri and other globular clusters

Abstract

X-ray bursts from bright sources in globular clusters of our galaxy show that at least 11 (of a total 13) of these sources are neutron stars. One of the low-luminosity X-ray sources in omega Cen is a neutron star accreting at a low rate. Together with the discoveries that M~15 contains two bright X-ray sources, and that 47 Tuc and NGC 6440 contain 2 and 5 low-luminosity X-ray sources with a neutron star, this indicates that the total number of binaries with neutron stars in globular clusters is higher than previously suspected. The discovery of very bright X-ray sources in globular clusters near other galaxies indicates that these clusters contain binaries with accreting black holes, and multiple bright X-ray sources. Dozens of low-luminosity X-ray sources have been discovered in omega Cen, NGC 6397, NGC 6752, NGC 6440, and a hundred in 47 Tuc. Accurate Chandra positions of these sources combined with HST observations have lead to unambiguous identifications with cataclysmic variables, RS CVn binaries, and millisecond radio pulsars.