The discovery of 13 s X-ray pulsations from the hydrogen depleted subdwarf O6 star binary HD49798

Abstract

We discovered strong ∼ 13 s X-ray pulsations in the Rosat PSPC light curve of HD49798, a 1.55 d single-component spectroscopic binary containing a hydrogen depleted subdwarf O6 star. We find no evidence for period changes during the ∼ 4 hr Rosat pointing. The source X-ray spectrum is extremely soft, with an unabsorbed 0.1-2 keV luminosity of a few ×10 32 erg s −1 (distance of 650 pc). A higher luminosity might be hidden in the EUV. Our results imply that the unseen companion is an accreting degenerate star, a white dwarf or, more likely, a neutron star. In either case HD49798 corresponds to a previously unobserved evolutionary stage of a massive binary system, after common envelope and spiral-in.