X-ray evidence for wind-wind collision in the Wolf-Rayet binary V444 Cygni

We report the results of three ASCA observations of the eclipsing Wolf-Rayet binary V444 Cyg (WN5+O6). These observations were obtained at orbital phases 0.0, 0.25, and 0.5, with the WN5 star in front at phase 0.0 and the O6 star in front at phase 0.5. Acceptable fits of the X-ray spectra using optically thin plasma models require at least two different temperature components, with a soft component at kT(1) approximate to 0.6 keV and a harder component at kT(2) approximate to 2 keV. The absorption of the hard component varies with orbital phase and is largest when the WN5 star is in front, whereas the X-ray luminosity of the hard component is at a minimum when the O6 star is in front. The high plasma temperature and variability with orbital phase suggest that the hard-component emission is caused by a colliding wind shock between the WN5 and O6 stars, with the shock most likely located near the surface of the O6 star. On the other hand, the soft-component emission at kT(1)approximate to 0.6 keV has nearly constant absorption and X-ray luminosity. The soft-component luminosity is L-x,L-1=(6-11)x10(32) ergs s(-1) (0.2-4 keV), implying L-x,L-1/L-bol similar to 10(-6) to 10(-7). This luminosity ratio and the soft-component temperature are similar to those of single massive stars, leading us to atf:attribute the soft emission to the individual O6 and WN5 components.