High energy emission powered by accreting companions of Be/gamma Cas stars

TL;DR

This study tests whether the hard X-ray emission characteristic of gamma Cas analogs can originate from accretion onto a white dwarf companion. Using 3D SPH simulations, the authors model the prototype gamma Cas with a WD and compare it to 59 Cyg, a Be star with an sdO companion, predicting disk structures and accretion-driven X-ray fluxes; they couple these results to the HDUST radiative-transfer code to forecast H-alpha emission and polarization in optical/UV bands. The results show that WD accretion can reproduce X-ray luminosities comparable to gamma Cas, while the sdO scenario yields substantially lower X-ray output; both systems develop Keplerian, disk-like accretion around the secondary, with HDUST-predicted H-alpha and polarization consistent with observations. The work highlights UV spectropolarimetry as a promising diagnostic for Be binaries and suggests future UV missions could decisively test the Be+WD accretion scenario as the origin of gamma Cas–like X-rays.

Abstract

The origin of the hard, bright X-ray emission that defines the gamma Cas analog class of Be stars remains an outstanding question in Be star literature. This work explores the possibility that the X-ray flux is produced by accretion onto a white dwarf companion. We use three-dimensional smoothed particle hydrodynamics simulations to model the prototype gamma Cas system assuming a white dwarf companion and investigate the accretion of the circumstellar material by the secondary star. We contrast these results to a model for 59 Cyg, a non-gamma Cas Be star system with a stripped companion. We find that the secondary stars in both systems form disk-like accretion structures with Keplerian characteristics, similar to those seen in the Be decretion disks. We also find that white dwarf accretion can produce X-ray fluxes that are consistent with the observed values for gamma Cas, while the predicted X-ray luminosities are significantly lower for the non-degenerate companion in 59 Cyg. In addition, using the three-dimensional radiative transfer code, HDUST, we find that these models produce H-alpha emission consistent with the observations for both gamma Cas and 59 Cyg, and that the predicted polarization degrees across optical and UV wavelengths are at detectable levels. Finally, we discuss the impact that future UV spectropolarimetry missions could have on our understanding of these systems.

Figures (9)

• Figure 1: Top-down snapshots of the sph simulation for $\gamma$ Cas (top) and 59 Cyg (bottom) at 75.0 P$_{\rm{orb}}$, 75.5 P$_{\rm{orb}}$, 75.6 P$_{\rm{orb}}$ and 75.9 P$_{\rm{orb}}$ (from left to right). The insets at the top left of each image show an enlarged view of the circumsecondary region. As explained in the text, $R_{\star}$ represents the equatorial radius of the Be star in each system. Images rendered using splashpri07.
• Figure 2: Disk scale heights (left), azimuthally averaged surface densities (center), and azimuthal velocities (right) for the $\gamma$ Cas model. The top row shows these quantities for the system using a grid centered on the primary star, while the bottom row shows the values for a grid centered on the secondary star. The dashed line indicates the theoretical value for each quantity. The solid vertical line indicates the position of the secondary star (top row) or primary star (bottom row). The dotted line represents the L1 point. Only the particles with negative specific energies with respect to each star are shown.
• Figure 3: Same as Fig. \ref{['fig:gCas_scaleheights']}, but for 59 Cyg.
• Figure 4: Radial velocity maps for $\gamma$ Cas (top) and 59 Cyg (bottom) models, measured with respect to the primary star. The dot at the center represents the primary star, while the secondary is represented by the black star and the contours represent surface density. The radial distances are in $R_{\star}$.
• Figure 5: Same as in Figure \ref{['fig:vrad_primary']}, but measured with respect to the secondary star, which is denoted as the black star at the center of each image.