Rosat X-Ray Observations of the Wr Stars HD 50896 (WN5) and γ Velorum (WC8+O9I)

Abstract

We present a preliminary analysis of multiple X-ray (0.1-2.5 keV) observations of HD 50896 and γ Velorum obtained with the ROSAT satellite. For HD 50896, our 8 observations show variability at the 30% level on timescales of ~ 1 day, together with larger (× 1.7) epoch-changes, but no evidence for rapid variability. No phase-dependent modulation is apparent on the 3^d.766 optical period. The mean PSPC spectrum gives kT = 0.28 keV, log N(H) = 20.6, and L_x = 3.8 × 10³² erg s^-1, and implies that the observed X-rays have undergone little absorption in the WN5 wind.

For γ Velorum, we have 13 observations secured over several cycles in the 78^d.5 binary period. At most binary phases, the X-ray emission is relatively constant, with kT ≃ 0.19 keV, log N(H) = 20.2, and L_x = 2.5 x 10³¹ erg s^-1. Near orbital phase 0.5, the X-ray emission is enhanced by a factor of 4, due almost entirely to an additional harder component with kT ≥ 2 keV. We believe this is due to X-ray emission produced in the collision of the two stellar winds.