1996A&A...310..577W

We present an analysis of 8 X-ray observations of HD 50896 obtained in our Guest Observer programmes with the XRT+PSPC on the ROSAT satellite, covering the energy range [0.1-2.5keV]. Our first observation, obtained in AO2, covered an integration time of 7ksec on 29 March 1992. This was followed by a further 7 PSPC observations, during AO3, at roughly daily intervals during 20-26 September 1992. The total PSPC count rates show evidence for variability at the ≤30% level on timescales of ≤1day, together with larger (x1.7) epoch-changes. We find no evidence for significant variations on very short timescales (≤1hour) previously reported from Einstein X-ray data. No `phase-dependent' modulation is apparent in the PSPC total count rates, or spectral fits, within the 3.766d optical period of the source. Single component spectral fitting (blackbody emission + photoelectric absorption) to the derived mean PSPC spectrum yields kT=0.28keV, and logN(H)=20.6. The latter is very close to the line-of-sight interstellar hydrogen column to HD 50896 of logN(H)_ism=20.7, implying that the observed X-ray emission from the source has undergone negligible absorption in the WN 5 wind. With an adopted distance of 1.8kpc, the mean X-ray luminosity in the PSPC energy range is 3.1x10³²erg/s, whilst the overall observed variations span the range [2.2-3.8]x10³²erg/s. Adopting a mass loss rate of 5x10^–5M_☉/yr, and a wind terminal velocity of v_∞=2000km/s, we estimate the 0.28keV X-ray emergent emission has a characteristic radius of ≥1000R_☉. A re-analysis of archival Einstein IPC X-ray [0.2-4kev] observations of HD 50896 at three different epochs confirms substantial variability (L_x~[3-9]x10³²erg/s), but shows no significant changes in spectral shape, for which kT∼0.4keV is derived - compatible with that deduced from our ROSAT data. We conclude that the observed PSPC and previous IPC X-ray emission from HD 50896 originates via mechanisms intrinsic to the WR wind, probably in shocked gas produced by radiatively-induced instabilities of the kind being predicted theoretically by Owocki and co-workers.