XRISM solves famous star’s 50-year mystery
European Space Agency and partner data from the X‑Ray Imaging and Spectroscopy Mission (XRISM) have resolved a decades‑long astronomical puzzle involving the bright northern star Gamma Cassiopeiae (γ Cas) by pinpointing the origin of its unusual high‑energy X‑ray emissions to an unseen companion star orbiting it. New observations showed that the plasma producing the intense X‑rays follows the orbital motion of a compact secondary object rather than emanating from the massive Be‑type star itself, confirming that material from γ Cas’s circumstellar disc is being accreted onto the companion. This finding concludes more than fifty years of uncertainty about the physical source of the star’s anomalous X‑ray signature.
The XRISM spacecraft, a collaboration between JAXA, NASA, and ESA, observed γ Cas using its high‑resolution Resolve spectrometer, which enabled measurements of X‑ray velocities associated with the hot plasma as it moved in sync with the compact object’s orbit. By contrasting the spectral motion against the expected behaviour of the primary star, scientists determined that the emissions arise from material falling toward a white dwarf companion, whose gravity strips and heats the gas to X‑ray‑emitting temperatures. These observations directly link the high‑energy X‑ray plasma not to the Be‑star itself but to accretion processes around its dense partner.
Identifying γ Cas as part of a Be‑type plus white dwarf binary system aligns with long‑held theoretical predictions about the population of high‑energy X‑ray emitters but lacked decisive observational confirmation until now. The result not only resolves a persistent astrophysical question about one of the sky’s most observed bright stars but also demonstrates XRISM’s capability to clarify the dynamics of extreme cosmic environments. Establishing this class of binary systems expands the framework for high‑energy astrophysics and sets the stage for future targeted investigations of similar objects across the Milky Way.
