Scientists identify companion behind a star's unexpected X-ray emissions

XRISM X-ray observations show the Be star gamma‑Cas is transferring material onto a compact companion, likely a white dwarf, which explains its strong X-ray emission. The team published the results March 24 in Astronomy & Astrophysics.

New observations with the XRISM X-ray spacecraft show that the bright Be star gamma‑Cas is losing material to a compact companion, likely a white dwarf. Gamma‑Cas lies about 550 light‑years away and was first noted in 1866 for an unusual hydrogen spectral feature. In the 1970s astronomers also discovered it emits unusually strong X-rays from very hot plasma. For decades researchers debated whether the X-rays came from magnetic interactions or from material falling onto an unseen companion. Key findings: - XRISM observations revealed a compact companion orbiting gamma‑Cas, interpreted as likely a white dwarf. - Gamma‑Cas is about 550 light‑years away, roughly 19 times the Sun’s size and up to 65,000 times as luminous. - The star’s X-rays are associated with plasma near 150 million degrees and are about 40 times brighter than a typical massive star. - More than 20 similar “gamma‑Cas‑like” Be stars have been identified by missions including XRISM, XMM‑Newton, Chandra and eROSITA. - The research was published March 24 in the journal Astronomy & Astrophysics. Summary: The new XRISM data favor the explanation that gamma‑Cas’s unusual X-rays arise from accretion onto a compact companion rather than magnetic interaction alone. Researchers say this identification allows the development of models tailored to this class of Be star systems and will inform updates to our understanding of binary evolution. The result builds on earlier XMM‑Newton work and applies to a growing set of similar high‑energy Be stars.