An international team of researchers led by astronomers from the University of Liège, using observations from the Japanese XRISM space telescope, has determined that mysterious X-rays from a star called Y Cassiopeia in the constellation Cassiopeia, which have puzzled astronomers for half a century, originate from a white dwarf orbiting the star.
The research team behind the discovery said that using the space telescope’s Resolve instrument to confirm the mysterious X-ray emissions source with an intensity and temperature deemed ‘incompatible’ with what one would expect from an ordinary, massive star like y Cas confirms the existence of a family of binary star systems that had been previously predicted but remained unidentified.
Mysterious Deep Space X-Rays Have Puzzled Astronomers for Half a Century
According to a statement announcing the discovery, y Cassiopeia, which was first discovered in 1866, is classified as a Be-type star. Described as “fast-rotating massive stars that regularly eject matter,” Be stars have a disc around them formed by this ejecta, which astronomers can spot at interstellar distances due to characteristic emissions in their optical spectrum.
In 1976, observations of these emissions yielded contradictory results, leading to a mystery that persisted until the University of Liège-led discovery. Specifically, y Can seemed to emit X-rays with a luminosity approximately 40 times greater than that of comparable massive stars, with plasma heated to over 100 million degrees and unusually rapid variability.
Since then, twenty additional objects with similar, mysteriously confounding data have been observed, resulting in the formation of an entirely new subclass of stars called ‘y Cas analogues.’ Over half of these unusual objects were spotted by astronomers at the University of Liège.
According to Yaël Nazé, an astronomer at ULiège, astronomers have proposed several possible explanations for the mysterious X-ray emissions.
“One of them involved local magnetic reconnection between the surface of the Be star and its disc,” the researcher explained. “Others suggested X-rays to be linked to a companion, whether a star stripped of its outer layers, a neutron star, or an accreting white dwarf.”
Although ULiège had already ruled out the first two types of companions due to contradictions between observations and theoretical predictions, their data left open the possibility of white dwarf stars and magnetic interactions. However, they cautioned, “no observation allowed to choose between them.”
Solving Mystery ‘Opens Up New Avenues of Research for the Years to Come’
To solve the longstanding mystery, the researchers conducted a new observation campaign focused on y Cas using the microcalorimeter Resolve instrument. Launched aboard the Japanese XRISM space telescope, Resolve is noted for providing spectra with “unrivaled precision.”
Because the target system has a full orbital period of 203 Earth days, the team conducted observations in December 2024, February, and June 2025. Nazé said those observations revealed that the signatures of the high-temperature plasma change velocity between the three observations, following the orbital motion of the white dwarf rather than that of the Be star,” the researcher continues.
“This shift was measured with high statistical reliability,” the scientists explained. “It is, in fact, the first direct evidence that the ultra-hot plasma responsible for the X-rays is associated with the compact companion, and not with the Be star itself.”
A further analysis of the Resolve data revealed that the spectral signatures were of moderate width, on the order of 200 km/s. The team said this measurement “effectively rules out the case of a non-magnetic white dwarf.” That’s because a white dwarf accretion occurs rapidly in the inner regions of that category of star’s disc, which produces very broad signatures.
Instead, the team determined that the best explanation for the mysterious X-rays is y Cas is magnetic, where “the disc is then truncated, and the magnetic field channels the accreting material towards its poles.” If confirmed by future observations, this would mean that y Cas and the twenty other stellar objects in its analogue subclass are Be + white dwarf binary systems where the objects share an orbit. The researchers said these objects have been predicted for a long time but were “never clearly identified” before now.
When discussing the implications of their findings, Nazé said that astronomers and astrophysicists will likely have to revise binary stellar evolution models, “particularly regarding the efficiency of mass transfer between components,” which would make it consistent with several other recent independent studies.
Understanding the evolution of binary systems is crucial for comprehending, for example, gravitational waves, as it is indeed massive binaries that emit them at the end of their lives,” the researcher explained. “Solving this mystery, therefore, opens up new avenues of research for the years to come!”
The study “Orbital motion detected in γ Cas Fe K emission lines” was published in Astronomy and Astrophysics.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.