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

TL;DR

This study tests whether the hard X-ray emission from the γ Cas-type Be star V750 Ara arises from accretion onto a white dwarf companion. Using deep Chandra/HETG and NuSTAR observations, the authors fit a cooling-flow plasma model to derive a very hot plasma ($T_{\max}\approx 2.6\times10^{7}$ K), an accretion rate of $\dot{M}\approx 1\times10^{-10}\,M_\odot\,\mathrm{yr}^{-1}$, and a WD mass of $M_{\rm WD}\approx 0.9\,M_\odot$, with emission-line widths around $\mathrm{FWHM}\sim 1000$ km s$^{-1}$ and flickering variability. Fe XXV/XXVI lines are detected, while Fe K fluorescence is weak in the high-resolution data but present in NuSTAR, consistent with geometry and disk-density effects. Collectively, the results support a WD-accretion origin for V750 Ara’s hard X-rays, aligning it with other γ Cas-type stars and providing quantitative constraints on accretion and line kinematics that inform models of Be-disk interactions with compact companions.

Abstract

We present 197 ks HETG and 95 ks NuSTAR spectra of the $γ\,$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$α$ 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.

Figures (3)

• Figure 1: NuSTAR light curve (top left) in $200\,$s (gray) and $5828\,$s bins (red). The larger bin size was chosen to match the period of the NuSTAR orbit. On the top right we show the power-spectral-density of the light curve which shows a slope indicative of red-noise -- flickering at amplitudes larger than expected from Poisson noise. We did not have enough sensitivity to reach the Poisson noise floor. Blue tic marks at the top indicate the NuSTAR orbital frequency and harmonics, none of which are evident in the transform. Bottom: the HETG light curve of dispersed events in $1\,\mathrm{ks}$ bins, with the observations concatenated in time order in alternating colors. The NuSTAR observation is between the two observations at about $95\,\mathrm{ks}$. Count rates are given in $\mathrm{s}^{-1}$. The power-spectral-density is a unitless normalized probability.
• Figure 2: HETG spectrum of V750 Ara in black, which is the merged HEG and MEG first orders, the HETG zeroth order in cyan, and the NuSTAR spectrum in blue (combined FPMA and FPMB). In red is a model using an absorbed cooling-flow model plus a Gaussian for a tentative Fe K fluorescence line. Features with significant detections are marked below the spectrum. The bottom panel shows the residuals in the form of the delta-Cash statistic ($\Delta C$), appropriate for Poisson distributed counts, normalized by its square root absolute value ($|\Delta C|^\frac{1}{2}$), to make it analogous to the commonly used normalized $\Delta\chi$ statistic. Colors are as in the upper panel.
• Figure 3: Line widths and offsets from Gaussian fits. Only features which provided meaningful constraints are shown. Confidence limits are $1\sigma$. H-like feature velocities are in reference to the emissivity weighted mean of the doublet's wavelengths. The Mg xi He-like resonance, intercombination, and forbidden lines had their relative positions constrained and a common width parameter. Widths are the intrinsic value required in addition to instrumental broadening.