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Multiple and variable X-ray sources in the globular clusters Cen, NGC 6397, NGC 6752, and Liller 1
Received 14 January 2000 / Accepted 13 April 2000
We detect stars from the Hipparcos and Tycho Catalogues in the field of view of observations with the ROSAT HRI of three globular clusters. We use the positions of these stars to reduce the systematic error in the positions of X-ray sources in the clusters to for Cen and NGC 6752, and for NGC 6397. We detect three X-ray sources in the core of 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 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.
Key words: Galaxy: globular clusters: individual: ! Cen, NGC6397, NGC6752, Liller 1 X-rays: stars
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Online publication: June 20, 2000