Multiple and variable X-ray sources in the globular clusters ω Cen, NGC 6397, NGC 6752, and Liller 1

We detect stars from the Hipparcos and Tycho Catalogues in the field of view of observations with the ROSAT HRI of three lobular clusters. We use the positions of these stars to reduce the systematic error in the positions of X-ray sources in the clusters to similar to 2 " for omega Cen and NGC6752, and 1 " for NGC 6397. We detect three X-ray sources in the core of omega Cen, and four in the core of NGC 6752; the data for the center of NGC 6397 may be fitted with five or six sources. Outside the cores, but within the half-mass radius of the clusters, we detect two sources in omega Cen, one in NGC 6397 and two in NGC 6752; these may or may not be cluster members. A ROSAT HRI observation of Liller 1 does not detect a low-luminosity source, at a limit below a detection with ASCA. We discuss the nature of the low-luminosity X-ray sources in globular clusters in the light of these new results.