Overview
- The peer‑reviewed study published Tuesday in Astronomy & Astrophysics uses XRISM’s high‑resolution Resolve spectrometer to tie γ Cas’s intense X‑rays to a hidden white dwarf companion.
- Three XRISM observations in December 2024, February 2025, and June 2025 tracked Doppler shifts in iron K‑line emission that match the 203‑day orbit of the low‑mass companion rather than the Be star.
- Line widths of about 200 km/s rule out a non‑magnetic accretion disc and point to a magnetic white dwarf that channels infalling gas along its field lines to the poles, where the gas heats and emits X‑rays.
- The finding identifies γ Cas and its analogues as Be + white dwarf binaries, which appear rarer and concentrated in higher‑mass Be stars, suggesting models must revise how efficiently mass moves between binary partners.
- XRISM, a JAXA mission with NASA and ESA, delivered the decisive spectra after years of groundwork by XMM‑Newton, Chandra, and eROSITA, resolving why γ Cas shines with plasma up to ~150 million K and about 40 times typical X‑ray brightness.