Astron. Astrophys. 359, 960-966 (2000)

X-ray and optical follow-up observations of the August 1998 X-ray transient in NGC 6440

F. Verbunt ¹, M.H. van Kerkwijk ¹, J.J.M. in't Zand ² and J. Heise <sup>2

1</sup> Astronomical Institute, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
² Space Research Organization Netherlands, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands

Received 14 March 2000 / Accepted 15 May 2000

Abstract

The BeppoSAX Wide Field Cameras detected a transient in NGC 6440 on 1998 Aug 22. ROSAT did not detect this source on 1998 Sep 8, indicating that the X-ray flux had decreased by a factor 400 at least, and/or that the X-ray spectrum had become appreciably softer. Analysis of archival ROSAT HRI data reveals two sources associated with NGC 6440; one of these may be the transient in quiescence. We have also obtained B and R images of NGC 6440 on 1998 Aug 26 and again on 1999 July 15, when the transient had returned to quiescence. Subtraction of these images reveals one object in the core which was brighter in B, but not in R, during the X-ray outburst. We estimate and on 1998 Aug 26, which makes it a viable candidate counterpart.

Key words: stars: individual: SAX J1748.9-2021, MX 1746-20 stars: neutron Galaxy: globular clusters: individual: NGC 6440 X-rays: stars

Send offprint requests to: F. Verbunt

Correspondence to: verbunt@phys.uu.nl

SIMBAD Objects

Contents

• 1. Introduction
• 2. X-ray observations
  • 2.1. The 1998 observation
  • 2.2. Earlier ROSAT observations: standard analysis
  • 2.3. Closer investigation of the central source
• 3. Optical observations
  • 3.1. Astrometry
  • 3.2. Variable sources
• 4. Discussion
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000
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