BeppoSAX broad–band observations of Gamma Cassiopeiae

We report broad-band X-ray measurements of the Be star γ Cassiopeiae by the BeppoSAX X-ray astronomy satellite. The observations took place on 1998 July, 18–23. The 0.1– 200 keV X-ray spectrum is reasonably well fit by an optically thin thermal plasma model of temperature 12.5±0.6 keV with significant residuals around 0.3 keV and 1 keV. The former is interpreted as the variable soft component reported by ROSAT, although there is no evidence for variability at the 5% level. For a blackbody interpretation, the fitted temperature is 100±320 13 eV, in agreement with the ROSAT value of 200± 10 eV. However, a MEKAL interpretation gives a significantly lower temperature of (48±11 eV). The fitted abundances are about half solar values, in agreement with previous measurements. At higher energies, the spectrum does not require non-thermal components and the observation of a line at 6.8 keV supports the ASCA interpretation of the source as an accreting white dwarf. Assuming a source distance of 188 pc, the bolometric luminosity in the 2–10 keV band is 6×1032 ergs s−1. Simultaneous optical measurements by the Wendelstein Observatory near Munich, indicate that the source continues to be in a late but rather normal Be phase, with no obvious signs of a transition to the Be-shell phase. The measured magnitudes at B, V and R wavelengths of 2.18±0.06, 2.23±0.02 and 2.36±0.03, respectively, confirm this.