Abstract
Abstract γ Cas and its dozen analogs comprise a small but distinct class of X-ray sources. They are early Be-type stars with an exceptionally hard thermal X-ray emission. The X-ray production mechanism has been under intense debate. Two competing ideas are (i) the magnetic activities in the Be star and its disk, and (ii) the mass accretion onto the unidentified white dwarf (WD). We adopt the latter as a working hypothesis and apply physical models developed to describe the X-ray spectra of classical WD binaries containing a late-type companion. Models of non-magnetic and magnetic accreting WDs were applied to γ Cas and its brightest analog HD 110432 using the Suzaku and NuSTAR data. The spectra were fitted by the two models, including the Fe fluorescence and the Compton reflection in a consistent geometry. The derived physical parameters are in a reasonable range in comparison to their classical WD binary counterparts. Additional pieces of evidence in the X-ray spectra—partial covering, Fe L lines, Fe i fluorescence—were not conclusive enough to classify these two sources into a subclass of accreting WD binaries. We discuss further observations, especially long-term temporal behaviors, which are important to elucidate the nature of these sources, if indeed they host accreting WDs.
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