Suzaku and NuSTAR X-Ray Spectroscopy of gamma Cas and HD110432

TL;DR

This study analyzes X-ray spectra of gamma Cas and HD110432 using Suzaku and NuSTAR data, testing accreting white dwarf models to understand their hard X-ray emission mechanisms.

Contribution

It applies physical accreting WD models to gamma Cas and HD110432, providing estimates of WD parameters and evaluating the accretion hypothesis for their X-ray emission.

Findings

WD mass and accretion rate are consistent with classical WD binaries.

Spectral features are inconclusive for classifying as accreting WD systems.

Further long-term observations are needed to confirm the accretion hypothesis.

Abstract

Gamma 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 gamma Cas and its brightest analog HD110432 using the Suzaku and NuSTAR data. The spectra were fitted by the two models, including the Fe I fluorescence and the Compton reflection in a consistent geometry. The derived physical parameters, such as the WD mass and mass accretion rate, 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, and Fe I fluorescence) were not conclusive enough to classify these two sources into a sub-class of accreting WD binaries. We discuss further observations, especially long-term temporal behaviors, which are important to elucidate the nature of these sources more if indeed they host accreting WDs.