X-ray emission from Wolf-Rayet stars

Seven bright Wolf-Rayet (WR) stars were observed with the Einstein X-ray observatory; four were detected. The 0.15-4 keV fluxes of the detected stars (and the upper limits for the others) are consistent with the ratio of X-ray to bolometric luminosities observed for other early-type stars. The X-ray spectra of WR stars are also similar to those of OB stars, with most of the photons emerging with energies less than 1 keV. These observations support the idea that X-ray emission in all hot stars is produced by shocks in the wind. The X-ray flux of one of the stars, HD 50896, varied by a factor of 2 within half an hour on two of the three days when it was observed. The possible origin of these variations is discussed; it is concluded that the companion of HD 50896 is either a black hole or a white dwarf remnant of a massive star.