EDP Sciences
Free access
Issue
A&A
Volume 380, Number 1, December II 2001
Page(s) 251 - 257
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20011396


A&A 380, 251-257 (2001)
DOI: 10.1051/0004-6361:20011396

ROSAT/Chandra observations of a bright transient in M 81

K. K. Ghosh1, D. A. Swartz2, A. F. Tennant3 and K. Wu4

1  NRC, NASA Marshall Space Flight Center, SD-50, Huntsville, AL 35812, USA
2  USRA, NASA Marshall Space Flight Center, SD-50, Huntsville, AL 35812, USA
    e-mail: doug.swartz@msfc.nasa.gov
3  Space Science Department, NASA Marshall Space Flight Center, SD-50, Huntsville, AL 35812, USA
    e-mail: allyn.tennant@msfc.nasa.gov
4  Mullard Space Science Laboratory, University College London, Holmbury St Mary, Surrey RH5 6NT, UK and School of Physics A28, University of Sydney, NSW 2006, Australia
    e-mail: kw@mssl.ucl.ac.uk

(Received 19 July 2001 / Accepted 4 October 2001 )

Abstract
We present a 10-year X-ray light curve and the spectra of a peculiar X-ray transient in the spiral galaxy M 81. The source was below the detection limit of ROSAT PSPC before 1993, but it brightened substantially in 1993, with luminosities exceeding the Eddington limit of a 1.5-$M_\odot$ compact accretor. It then faded and was not firmly detected in the ROSAT HRI and PSPC observations after 1994. The Chandra image obtained in 2000 May, however, shows an X-ray source at its position within the instrumental uncertainties. The Chandra source is coincident with a star-like object in the Digitized-Sky-Survey. A Hubble image suggests that the optical object may be extended. While these three observations could be of the same object, which may be an X-ray binary containing a black-hole candidate, the possibility that the ROSAT and Chandra sources are two different objects in a dense stellar environment cannot be ruled out. The Hubble data suggests that the optical object may be a globular cluster yet to be identified.


Key words: X-rays: binaries -- X-rays: galaxies -- black hole physics -- stars: binaries: close -- globular clusters: general

Offprint request: K. K. Ghosh, kajal.ghosh@msfc.nasa.gov

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