The observation of the binary HD 49798 / WGA J0648.0-4418 in X-rays has supplied the first direct information on the secondary star, which allowed us to characterize this binary much more accurately than before. The detection of X-ray pulsations, revealing the compact nature of the secondary star, enabled us to constrain the possible mass range of both the secondary and the subdwarf. The mass of the subdwarf must be less than , and is in the range for a white dwarf companion of .
We reconstructed the evolutionary history of the system and conclude that at present the subdwarf is in a phase of shell helium burning and has a degenerate CO core of . It is the core of a progenitor with a mass probably between 4 and which shed its envelope in a common envelope event when it was on the early AGB.
The X-ray spectrum with the large pulsed soft component, its very good resemblance with the spectra of the intermediate polars, and the birthrate considerations, they all point in the direction of a white dwarf companion, and make it very unlikely that it is a neutron star. The X-ray luminosity and pulsations are explained by a magnetized white dwarf, accreting matter from the wind of the subdwarf. This model may be tested by a better determination of the wind parameters of the subdwarf and by further X-ray observations.
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998