Astron. Astrophys. 359, 573-585 (2000)

Doubling the number of Be/X-ray binaries in the SMC ^*

F. Haberl and M. Sasaki

Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany

Received 2 February 2000 / Accepted 14 April 2000

Abstract

A correlation of X-ray source and emission-line object catalogues in the Small Magellanic Cloud (SMC) shows that more than two thirds of the optically identified Be stars in Be/X-ray binaries are found as emission-line objects in the catalogues. On the basis of this result we propose up to 25 X-ray sources mainly from recent ROSAT catalogues as new Be/X-ray binaries and give their likely optical counterparts. Also for the five yet unidentified X-ray pulsars in the SMC we propose emission-line stars as counterparts. This more than doubles the number of known high mass X-ray binary systems in this nearby galaxy. The spatial distribution of the new candidates is similar to that of the already identified Be/X-ray binaries with a strong concentration along the SMC main body and some systems in the eastern wing. The new candidates contribute mainly to the low-luminosity end of the X-ray luminosity distribution of Be/X-ray binaries. A comparison with the luminosity distribution in the Milky Way reveals no significant differences at the high-luminosity end and the large number of low-luminosity systems in the SMC suggests that many such systems may still be undetected in the Galaxy. The overall ratio of known Be to OB supergiant X-ray binaries in the SMC is an order of magnitude larger than in the Galaxy, however, might show spatial variations. While in the eastern wing the ratio is comparable to that in the Galaxy no supergiant X-ray binary is currently known in the main body of the SMC. Possible explanations include a different star formation history over the last My.

Key words: galaxies: Magellanic Clouds galaxies: stellar content stars: variables: general X-rays: stars

* Figs. 3-6 are only available electronically here with the On-Line publication

Send offprint requests to: F. Haberl (fwh@mpe.mpg.de)

SIMBAD Objects

Contents

• 1. Introduction
• 2. X-ray sources and emission-line stars
  • 2.1. Supersoft sources
  • 2.2. Optically identified Be/X-ray binaries
  • 2.3. Optically unidentified X-ray pulsars
  • 2.4. New Be/X-ray binary candidates
  • 2.5. Chance coincidences
• 3. ASCA binary pulsar candidates
• 4. Optical identifications
• 5. The Be/X-ray binary population of the SMC
• 6. Notes to individual sources
  • 6.1. Supersoft sources
  • 6.2. Optically identified Be/X-ray binaries
  • 6.3. Optically unidentified X-ray pulsars
  • 6.4. New Be/X-ray binary candidates
• 7. Summary
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: July 7, 2000
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