Volume 488, Number 3, September IV 2008
|Page(s)||921 - 933|
|Section||Galactic structure, stellar clusters, and populations|
|Published online||01 July 2008|
X-ray and optical observations of M55 and NGC 6366: evidence for primordial binaries
Astronomical Institute, Utrecht University, PO Box 80 000, 3508 TA Utrecht, The Netherlands
2 Physics Department, McGill University, Montreal, QC H3A 2T8, Canada e-mail: firstname.lastname@example.org
3 Astronomy Department, UC Berkeley, 601 Campbell Hall, Berkeley, CA 94720-3411, USA
4 Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195, USA
5 Kavli Institute for Astrophysics and Space Research, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
Accepted: 26 June 2008
We present Chandra X-ray Observatory ACIS-S3 X-ray imaging observations and VLT/FORS2 and Hubble Space Telescope optical observations of two low-density Galactic globular clusters; NGC 6366 and M 55. We detect 16 X-ray sources with 0.5–6.0 keV luminosities above erg s-1 within the half-mass radius of M 55, of which 8 or 9 are expected to be background sources, and 5 within the half-mass radius of NGC 6366, of which 4 are expected to be background sources. Optical counterparts are identified for several X-ray sources in both clusters and from these we conclude that 3 of the X-ray sources in M 55 and 2 or 3 of the X-ray sources in NGC 6366 are probably related to the cluster. Combining these results with those for other clusters, we find the best fit for a predicted number of X-ray sources in a globular cluster , where Γ is the collision number and Mh is (half of) the cluster mass, both normalized to the values for the globular cluster M4. Some sources tentatively classified as magnetically active binaries are more luminous in X-rays than the upper limit of of such binaries in the solar neighbourhood. Comparison with XMM and ROSAT observations lead us to conclude that the brightest X-ray source in M 55, a dwarf nova, becomes fainter in X-rays during the optical outburst, in accordance with other dwarf novae. The brightest X-ray source in NGC 6366 is a point source surrounded by a slightly offset extended source. The absence of galaxies and Hα emission in our optical observations argues against a cluster of galaxies and against a planetary nebula, and we suggest that the source may be an old nova.
Key words: Galaxy: globular clusters: individual: NGC 6366 and M 55
© ESO, 2008
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