Orbital motion detected in gamma Cas Fe K emission lines

Abstract

A subset of Be stars, typified by the naked-eye star gamma Cas, exhibits unusually bright and hard X-ray emission, the origin of which has remained debated for five decades. We performed high-resolution X-ray spectroscopic monitoring of gamma Cas with the Resolve instrument aboard the X-Ray Imaging and Spectroscopy Mission (XRISM). X-ray lines from the ultra-hot plasma and fluorescence from cooler material exhibit Doppler shifts consistent with orbital motion, not of the Be star itself, but of its low-mass companion (previously shown to be a white dwarf). This first evidence of orbital motion for the hard X-ray emitting plasma uniquely links it to the scenario of accretion onto the white dwarf companion. The modest line broadening further indicates that fluorescence occurs on the white dwarf surface and excludes X-ray generation in the inner parts of an accretion disc. Our findings identify gamma Cas and its analogues as the previously elusive, but long predicted class of binaries composed of a Be and a white dwarf. Identifying the origin of the hard X-rays from gamma Cas and its analogues, which represent about 10% of early-type Be stars, provides a key input for population synthesis models of massive binary evolution.

Figures (6)

• Figure 1: H$\alpha$ line during XRISM observations naz25. Because the emission arises from the decretion disc surrounding the Be star, it shifts with the Be star's orbital motion.
• Figure 2: Resolve spectra over the full spectral range (7.5 eV per bin or 15-channel grouping). The three components of the iron complex at 6.4--7.0 keV are prominent, and additional fainter lines of other metals are also visible.
• Figure 3: Observed profile of the three iron features in the $\gamma$ Cas spectrum: fluorescence at 6.4 keV (second panel) and Fe xxv and Fe xxvi contributions (third and fourth panels, respectively). For comparison, the stationary calibration line of Mn K$\alpha$ in the three observations is also shown in the first panel. In these panels, velocities are calculated using rest energies of 5.89879, 6.40546, 6.70042, and 6.97317 keV (from top to bottom), and green tick marks indicate the main line components. The observation at maximum Be star orbital velocity (drawn in red) shows blueshifted iron lines relative to the observation at minimum Be star orbital velocity (drawn in blue). Typical 1-$\sigma$ error bars are shown for the latter observation.
• Figure 4: Shifts (top) and widths (bottom) of the iron features measure on $\gamma$ Cas XRISM spectra. Fluorescence lines are shown as black dots, ionised lines with green stars. As in Table \ref{['tab:rv']}, the error bars are 1-$\sigma$ uncertainties. In the top panel, all velocities are relative to that at the first observing phase and are compared to the expected orbital motion of both the Be star and its white dwarf companion, demonstrating that the iron lines follow the white dwarf motion. For comparison, small violet points indicate velocities measured on the H$\alpha$ line, which is typically associated with the Be disc, at the time of the X-ray campaign naz25.
• Figure 5: Line profile modelled at the Resolve resolution including all subcomponents of Fe ivpal03. Profiles are shown for the three orbital phases of the observations and three different scenarios: from top to bottom: accretion onto a magnetic white dwarf, accretion onto a non-magnetic white dwarf, and interaction between the Be star and its disc (see rau25). The colour convention follows Figs. \ref{['fig:optlines']} and \ref{['fig:linesx']}.