The Changing X-Ray Properties of γ Cas

Abstract

The origin of the hot X-ray emission from γ Cas has been a subject of long debate, with some authors advocating that it arises from a supposed undetected degenerate companion. Parmar et al. (1996) concluded that X-ray properties alone limited one’s ability to understand this source. Thus, Smith et al. (1998, “SRC”) obtained ≈1 day’s simultaneous X-ray and UV spectroscopic and photometric coverage with the RXTE/PCA and HST/GHRS space-borne instruments on 14-15 March 1996. The observed 2–10 keV X-ray light curve was found to have two distinct components; short duration bursts (‘shots)’ superimposed on a slowly varying background, termed the “basal” flux, which modulates on a timescales of hours. SRC found two maxima in the RXTE light curve separated by 10 hours. These features coincided with the appearance of two “dips” in the UV continuum light curve sampled near λ1400. From these light curves and other arguments, SRC concluded that γ Cas’s X-ray and UV continuum flux originated near the surface of the star and were modulated in intensity as the star rotated with a period of 1.123 days. This period was deduced by phasing of the X-ray minima obtained from our RXTE data with ASCA observations taken 10 days earlier and from comparisons of UV minima observed with the GHRS with similar minima seen in IUE observations taken two months earlier.