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Session 93 - Binary and Variable Stars.
Oral session, Friday, January 09
To address whether the well-known, 10^8K X-rays from \gammaCas are emitted from the Be star or a degenerate companion, we have conducted simultaneous observations with the RXTE/PCA and HST/GHRS satellites over nearly a full day on 1996 March 14-15. A quasi-continuum light curve formed at \lambda1400 shows two \sim1% dips separated by about 10 hours. The X-ray flux curve is dominated by two variabilities, slow undulations on the rotational timescale and a rapid one with a \sim1/f power spectrum. The slow X-ray undulations shows two peaks that coincide in time with two UV light dips, which suggests the X-rays arise from rotational modulation. The stability of this component has been verified by phasing our data with contemporaneous ASCA data, resulting in a period of 1.125 days. Also, IUE observations two months earlier show flux and color variations near the phases of the GHRS observations. These match if the period is adjusted to 1.123 days and we take this as the star's rotational period.
The X-ray emission consists of a ``basal" component, representing the minimum flux observed at any phase, and of rapidly varying ``shots" with lifetimes ranging from \le10 s to several minutes. The shots have a slightly harder energy distribution, so these two components are probably not emitted cospatially. The average symmetric profile and the short lifetimes of the shots indicates they are formed in a high density region. From a simple flare model, we show that the site of the shots has a density of \ge10^14 cm^-3 and a scale of \le10^4 km. Most of the shot energy is not directly radiated but rather expands into a basal-emitting "canopy" with a density of \le10^11 cm^-3 and a scale \le0.1R_*. These results argue that \gamma Cas is a member of a new group of upper main sequence stars which continuously flare in X-rays.
Program listing for Friday