ASCA spectroscopy of the hard X-ray emission from the colliding wind interaction in gamma[SUP]2[/SUP] Velorum

We discuss an ASCA observation of the eccentric WC8+O7.5III binary gamma[SUP]2[/SUP]Velorum near apastron. The X-ray spectrum is compared with two previous observations obtained when the system was near periastron. All three spectra display a hard-emission component that undergoes strong variability over the orbital cycle. The properties of the hard X-ray emission of gamma[SUP]2[/SUP]Vel are constrained by taking into account the contribution from contaminating soft X-ray sources in the vicinity of gamma[SUP]2[/SUP]Vel. We find that the observed variations are in qualitative agreement with the predictions of colliding wind models. We investigate for the first time the effect of uncertainties in the chemical composition of the X-ray emitting plasma on our understanding of the high-energy properties of the wind interaction region. Our results indicate that these uncertainties significantly affect the derived shock temperature and absorption column, but play a smaller role in determining the intrinsic X-ray luminosity of the colliding wind zone. We further find that the intrinsic luminosity from the hard X-ray component in gamma[SUP]2[/SUP]Vel does not follow the 1/D distance relation expected from simple models of adiabatic shocks.