NONTHERMAL X-RAY-EMISSION FROM WINDS OF OB SUPERGIANTS

A new model is presented for the hard X-ray emission from early-type stars. X-rays harder than 2 keV can be produced through the inverse Compton mechanism when the stellar UV photons are scattered by relativistic electrons accelerated by shocks in the wind near the star. The emerging X-ray spectra are power laws (F(v) oc E-1/2) and fit hard components of the Einstein SSS spectra of three Orion OB supergiants quite well. The model uses a revised version of Lucy's shock model to fit the observed soft (thermal) X-ray component; this determines the wind and shock parameters which in turn can be used to calculate the hard X-ray spectra with only one free parameter. By comparison with the observations, our model predicts that the surface magnetic fields of early-type supergiants are only a few gauss, and the acceleration efficiency of electrons is comparable to that of ions near the shock front. Both of these results may have astrophysical significance in other circumstances. We emphasize that the idea proposed in this paper can be applied to all the early-type stars and that we limit its application to OB supergiants in this paper only because some of jour simplifying approximations break down in very high density or very low density winds. It is clear from our work that we can only understand the winds of early-type stars by understanding the physics of both radiatively driven shocks and the particles accelerated by those shocks.