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Astron. Astrophys. 358, 910-922 (2000)
Multiple and variable X-ray sources in the globular clusters ![[FORMULA]](img1.gif)
Cen, NGC 6397, NGC 6752, and Liller 1
F. Verbunt 1 and
H.M. Johnston 2
1 Astronomical Institute, P.O.Box 80000, 3508 TA Utrecht, The Netherlands
2 Anglo Australian Observatory, P.O. Box 296, Epping, NSW 2121, Australia
Received 14 January 2000 / Accepted 13 April 2000
Abstract
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
![[FORMULA]](img2.gif)
for
![[FORMULA]](img3.gif)
Cen and NGC 6752, and
![[FORMULA]](img4.gif)
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
Send offprint requests to: F. Verbunt
Correspondence to: verbunt@phys.uu.nl
Contents
- 1. Introduction
- 2. Observations and data reduction
- 3. Cen
- 4. NGC 6397
- 4.1. Source list and membership
- 4.2. The central sources
- 4.3. The earlier observations
- 4.4. Sources not related to the cluster
- 4.5. Discussion
- 5. NGC 6752
- 6. Liller 1
- 7. Summary and discussion
- Acknowledgements
- References
© European Southern Observatory (ESO) 2000
Online publication: June 20, 2000
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