X-ray variability in V444 Cygni: Evidence for colliding winds?

Phase-resolved ROSAT observations of the soft X-ray flux from the W-R + O star binary V444 Cyg confirm the orbital dependence of the flux suggested by Einstein IPC observations, showing a drop in flux around primary eclipse, when the W-R star is in front of the O star. The observed X-ray variability can be modeled as a wind eclipse of an X-ray source by the Wolf-Rayet wind. If most of the X-rays from the system are produced in a region of shock-heated gas formed by the wind collision between the two stars, then the shocked gas has a large physical extent (r similar to 140R(WR)) if the observed variability is produced solely by the eclipse of the region by the W-R wind. However, since the O star makes a significant contribution to the total X-ray flux from the system, the actual size of the shocked interaction region is probably much smaller. We discuss possible origins for the X-ray emission and conclude that it is probable that at least a fraction of the observed X-ray emission comes from the wind collision.