Astronomers have identified a hidden white dwarf companion as the source of extreme X-rays from the naked-eye star Gamma Cassiopeiae. Observations from Japan's XRISM space telescope revealed the companion's orbital motion through shifting plasma velocities. The discovery confirms a long-predicted class of binary star systems.
Gamma Cassiopeiae, a bright star visible to the naked eye in the constellation Cassiopeia, has puzzled scientists since 1976 with its unusually intense and hot X-ray emissions—about 40 times stronger than those from similar massive stars, with plasma exceeding 100 million degrees Kelvin. Classified as the first Be-type star in 1866 by Italian astronomer Angelo Secchi, it rapidly spins and ejects material into a surrounding disk. Around 20 similar stars, known as γ Cas analogues, have since been found, with researchers at the University of Liège identifying more than half. Yaël Nazé, an astronomer at the University of Liège, noted that prior theories included magnetic reconnection on the star's surface or companions like stripped stars, neutron stars, or accreting white dwarfs, but observations ruled out all but magnetic activity or a white dwarf. Precise data from the Resolve instrument on Japan's XRISM space telescope, collected in December 2024, February 2025, and June 2025, tracked the full 203-day orbit. The spectra showed high-temperature plasma signatures shifting velocity with the companion's motion, not the Be star's—direct evidence linking X-rays to the white dwarf. Spectral widths around 200 km/s indicate a magnetic white dwarf, where its field channels disk material to the poles. Led by University of Liège researchers including Nazé, Masahiro Tsujimoto, Gregor Rauw, and Sean J. Gunderson, the findings appear in Astronomy & Astrophysics. They confirm Be stars paired with white dwarfs—a predicted but unobserved binary type—affecting about 10% of massive Be stars. Nazé said this discrepancy calls for revising binary evolution models, especially mass transfer efficiency, aligning with recent studies and aiding understanding of gravitational waves from massive binaries.