Chandra/HETG and NuSTAR Observations of V750 Ara, a $\gamma\,$Cas-type Star

Abstract

We present 197 ks HETG and 95 ks NuSTAR spectra of the $\gamma\,$Cas-type object V750 Ara. The high-resolution X-ray spectra show that the target is similar to other objects of this class. Data are interpreted under the assumption that the X-rays come from an accreting white dwarf, and our analysis implies an accretion rate of about $3\times10^{-11}M_\odot\mathrm{yr}^{-1}$. Emission lines are weak, and predominantly from hydrogen-like ions: Mg XII, Si XIV, and S XVI. H-like and He-like Fe are both present, but Fe K$\alpha$ fluorescence is weak, being significantly detected only in the NuSTAR spectrum, but was not obviously detected in the HETG dispersed or zeroth-order spectra. The flux was variable above a level expected by Poisson statistics. There were no significant changes in the spectral hardness, though we are limited by lack of soft signal below 1 keV. Emission lines of Mg and Si were strong enough to measure velocity offsets and widths which were found to be marginally inconsistent. The H-like Mg line is consistent with instrumental broadening only, but shows a 300 km/s blueshift. He-like Mg and H-like Si lines have no significant shift in velocity but are broadened by about 1000 km/s. This suggests either different physical origins or velocity structure differing with plasma temperature.