Orbital motion detected in gamma Cas Fe K emission lines

TL;DR

High-resolution X-ray spectroscopy of gamma Cas reveals orbital motion of its white dwarf companion's plasma, confirming accretion as the source of its hard X-ray emission and identifying a new class of Be-white dwarf binaries.

Contribution

First direct evidence of orbital motion in gamma Cas's X-ray emitting plasma, linking it to accretion onto a white dwarf companion and identifying a new binary class.

Findings

Doppler shifts indicate orbital motion of plasma

Fluorescence occurs on white dwarf surface

Gamma Cas is part of a Be-white dwarf binary class

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.