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Astron. Astrophys. 317, 815-822 (1997)

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The nature of the bright subdwarf HD 49798 and its X-ray pulsating companion

B.C. Bisscheroux 1, O.R. Pols 2, P. Kahabka 1, 3, T. Belloni 1, 3 and E.P.J. van den Heuvel 1, 3

1 Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 Institute of Astronomy, Madingly Road, Cambridge, CB3 0HA, UK
3 Center for High Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Received 9 April 1996 / Accepted 14 June 1996


An analysis of the observed properties of the sub-dwarf O6 close binary system HD 49798 and its 13.18 s ultrasoft X-ray pulsating companion (WGA J0648.0-4418) is presented. On evolutionary grounds we show that the subdwarf must have a degenerate CO core and is in the phase of shell helium burning, which explains its high luminosity. The subdwarf, which probably has a mass between [FORMULA], is the descendant of a massive asymptotic giant branch star that lost its hydrogen-rich outer layers in a common-envelope event. We show that all observations are consistent with the X-ray source being a weakly magnetized massive white dwarf which is accreting matter from the wind of its subdwarf companion. We exclude a neutron star companion on the ground of (1) the ultrasoft spectrum of the X-ray source; (2) the very close resemblance of the X-ray spectrum and luminosity with that of the soft intermediate polars; (3) the relative closeness of HD 49798, which implies a Galactic birthrate of such systems that is much larger than that of binary pulsars with a massive white dwarf companion, the natural des-cendants of systems like HD 49798 if the pulsar in this system were a neutron star.

Key words: stars: individual: (HD 49798) – binaries: close – stars: evolution – subdwarfs – white dwarfs – X-rays: stars

Send offprint requests to: Bart Bisscheroux (bartb@astro.uva.nl)

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998