High-Energy Properties of the Enigmatic Be Star Gamma Cassiopeiae

Abstract

We present the results of a broad-band X-ray study of the enigmatic Be star Gamma Cassiopeiae (herein γ Cas) based on observations made with both the Suzaku and INTEGRAL observatories. γ Cas has long been recognized as the prototypical example of a small subclass of Be stars with moderately strong Xray emission dominated by a hot thermal component in the 0.5-12 keV energy range (L x ≈ 10 32 − 10 33 erg s −1 ). This places them at the high end of the known luminosity distribution for stellar emission, but several orders of magnitude below typical accretion powered Be X-ray binaries. The INTEGRAL observations spanned an 8 year baseline and represent the deepest measurement to date at energies above ∼ 50 keV. We find that the INTEGRAL data are consistent within statistics to a constant intensity source above 20 keV, with emission extending up to ∼100 keV and that searches for all of the previously reported periodicities of the system at lower energies led to null results. We further find that our combined Suzaku and INTEGRAL spectrum, which we suggest is the most accurate broad-band X-ray measurement of γ Cas to date, is fitted extremely well with a thermal plasma emission model with a single absorption component. We found no compelling need for an additional non-thermal high-energy component. We discuss these results in the context of a currently favored models for γ Cas and its analogs.