ASCA X-Ray Observations of Gamma Cassiopeiae

Abstract

We present the results of X-ray observations of γ Cassiopeiae made with the ASCA satellite. Since the discovery of X-ray emission from this system, several possible models for the origin of X-rays have been proposed, including the corona of γ Cas, itself, a binary neutron-star companion, and a binary white-dwarf companion. However, there has been no clear way to distinguish which, if any, of these models is correct. In this paper we exploit the spectral resolution of the ASCA SIS detectors which, for the first time, enables the iron-line emission from this system to be studied in detail. The line and continuous spectrum of γ Cas is compared with the X-ray spectra of hot coronae of early type stars, low-mass X-ray binaries, high-mass X-ray binaries, and white-dwarf binaries. The observed properties of rapid fluctuations 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 the X-rays, we estimated the luminosity of this system as a function of the binary separation using recent measurements of the circumstellar environment of γ Cas. Despite previous arguments that the X-ray luminosity is too large to be accounted for by accretion onto a white dwarf, we find that such a model is, in fact, consistent with the observed properties of the system.