X-ray emission from colliding wind binaries

I present new evidence of X-ray production by wind collisions in early-type binary systems by looking for phase-dependent changes in the observed X-ray emission. Newly derived X-ray lightcurves for 18 O-type binaries indicate that variability is far more common for O-type binaries than for single O stars. Four of the 18 systems studied show clear evidence of phase-dependent variability, suggesting that a large fraction of the X-ray emission is produced in a localized wind collision region. I examine 3 important WR + O systems, WR140, gamma(2) Velorum and V444 Cygni, and show that the observed X-ray variations are consistent with the predictions of modern colliding wind models.