ASCA X-RAY OBSERVATIONS OF GAMMA-CASSIOPEIAE

We present the results of X-ray observations of gamma Cassiopeiae made with the ASCA satellite. Since the discovery of X-ray emission from t his system, several possible models for the origin of X-rays have been proposed, including the corona of gamma Gas, itself, a binary neutron -star companion, and a binary white-dwarf companion. However, there ha s been no clear way to distinguish which, if any, of these models is c orrect. In this paper we exploit the spectral resolution of the ASCA S IS detectors which, for the first time, enables the iron-line emission from this system to be studied in detail. The line and continuous spe ctrum of gamma Cas is compared with the X-ray spectra of hot coronae o f early type stars, low-mass X-ray binaries, high-mass X-ray binaries, and white-dwarf binaries. The observed properties of rapid fluctuatio ns on timescales down to 10 s and a spectrum with a temperature of 10. 6 keV, together with iron line emission, are very similar to those of binary white-dwarf systems. Assuming a white-dwarf binary origin for t he X-rays, we estimated the luminosity of this system as a function of the binary separation using recent measurements of the circumstellar environment of gamma Cas. Despite previous arguments that the X-ray lu minosity is too large to be accounted for by accretion onto a white dw arf, we find that such a model is, in fact, consistent with the observ ed properties of the system.