Astronomers have identified the source of unusual X-rays from the bright star gamma-Cas as a hidden white dwarf companion siphoning material from it. High-resolution data from the XRISM space mission confirmed that the emissions arise from matter heating up as it falls onto the unseen star. The discovery ends a puzzle that has intrigued scientists since the 1970s.
The star gamma-Cas, visible to the naked eye in the W-shaped constellation Cassiopeia, has puzzled astronomers for decades. First noted in 1866 by Italian astronomer Angelo Secchi for its unusual bright hydrogen emission line, it helped define the class of Be stars. These hot, blue-white stars eject material into spinning discs that cause brightness variations. In the 1970s, strong X-rays from plasma at 150 million degrees added to the enigma, far hotter than expected for such a star alone. Observatories like ESA's XMM-Newton, NASA's Chandra, and eROSITA identified about two dozen similar gamma-Cas-type systems among Be stars. Yaël Nazé of the University of Liège, Belgium, led the study using XRISM's Resolve spectrometer. The observations revealed that the hot plasma moves in sync with the orbit of an unseen white dwarf companion, confirming accretion as the X-ray source. Nazé said, “There has been an intense effort to solve the mystery of gamma-Cas across many research groups for many decades. And now, thanks to the high-precision observations of XRISM, we have finally done it.” She added, “The previous work using XMM-Newton really cleared the way for XRISM, enabling us to eliminate numerous theories and prove which of the last two competing theories was correct. It's extremely satisfying to have direct evidence to solve this mystery at long last!” The finding resolves a key debate between magnetic disc interactions and companion accretion. It also prompts new questions about binary star evolution, particularly why such pairings with massive Be stars are rarer than expected. Alice Borghese, an ESA Research Fellow in high-energy astrophysics, noted, “It's incredible to see how this mystery has slowly unfolded over the years. XMM-Newton did so much of the groundwork... And now with the next generation of advanced instrumentation, XRISM has brought us over the finish line.” Matteo Guainazzi, ESA's XRISM Project Scientist, highlighted the international collaboration behind the result.