Research led by astronomers at the University of Liège in Belgium has identified the origin of the mysterious X-ray emissions from the star γ Cassiopeia (better known by its nickname “γ Cas”), located in the Cassiopeia constellation. Based on observations made by the Japanese XRISM space telescope, scientists showed that the extreme radiation is produced by a magnetic white dwarf that orbits the star, and not by γ Cas itself, as some hypotheses suggested, putting an end to a 50-year mystery.
The details of the study were published this Tuesday, March 24, in a scientific article in the journal Astronomy & Astrophysics.
The discovery, in addition to solving an enigma that persisted for almost half a century, also demonstrates the existence of a class of binary systems that, until then, it had only been predicted theoretically.
Visible to the naked eye, γ Cas was already known since the 19th century as the first identified Be-type star. These stars are very massive and spin rapidly, expelling matter and forming disks around them. However, since 1976, observations have revealed unusual behavior: γ Cas emits X-rays with an intensity approximately 40 times greater than that of similar stars, in addition to presenting plasma with temperatures greater than 100 million degrees and extremely rapid variations.
“Science has proposed several scenarios to explain this emission,” says astronomer Yaël Nazé, professor at the University of Liège in Belgium and co-author of the study, in a press release. “One of them involved a local magnetic reconnection between the surface of the Be star and its disk. Others suggested that the (increase in the mass of a space object)”.
Even after decades of study and the identification of about 20 similar objects, so-called “γ Cas analogs,” no hypothesis had been conclusively proven. The answer finally came with the instrument Resolvea high-precision microcalorimeter installed at XRISM, capable of analyzing X-ray spectra in unprecedented detail.
The team conducted three observation campaigns between December 2024 and June 2025, covering the entire orbital period of the binary system, approximately 203 days. The data provided conclusive evidence: The spectral signatures of the hot plasma varied in speed with time, following the orbital motion of the companion star.
“The spectra revealed that the high-temperature plasma signals change speed between the three observations, following the orbital motion of the white dwarf rather than that of the Be star,” explains the researcher. This change was measured with high statistical reliability. In fact, it is the first direct evidence that the ultra-hot plasma responsible for the X-rays is associated with the compact companion star, and not with the Be star itself.
Furthermore, analysis of the width of the spectral lines, which travel at speeds of approximately 200 km/s, ruled out the scenario of a non-magnetic white dwarf. Instead, the data indicate the presence of a significant magnetic field channeling the accreting material.
Based on these observations, the researchers propose a clear model: the Be star expels material that forms a disk around it; Some of this material is captured by the white dwarfcreating a second accretion disk. The compact object’s magnetic field directs this flow toward its poles, where the energy is released in the form of X-rays.
This discovery resolves the case for γ Cas and, at the same time, confirms the existence of a population of binary systems composed of Be-type stars and accreting white dwarfs, a class predicted decades ago, but never precisely identified.
However, the results also call into question established theoretical models. Observations indicate that these systems represent approximately 10% of Be stars and are mainly associated with the most massive ones, in contrast to predictions that pointed to a larger population composed of lower mass stars.
“This discrepancy suggests a revision of binary evolution models, in particular with respect to the efficiency of mass transfer between components, a conclusion that is consistent with several recent independent studies,” notes Nazé. “Solving this mystery, therefore, opens new avenues of research for years to come.”
The researcher also highlights the broader importance of the discovery: “Understanding the evolution of binary systems is crucial to understanding, for example, gravitational waves, since it is precisely massive binary systems that emit them at the end of their useful life.”
*By Arthur Almeida.
