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Observations from the XRISM telescope showed that the extreme X-rays of γ Cas come from a magnetic white dwarf orbiting the star
The star γ Cas had the origin of its intense X-ray emissions identified by astronomers from the University of Liège using data from the XRISM telescope, solving a 50-year mystery and confirming a binary class predicted only in theory.
Discovery ends a five-decade mystery
A team led by astronomers from the University of Liège in Belgium identified the origin of the extreme X-ray emissions associated with the star γ Cassiopeia, known as γ Cas, located in the constellation Cassiopeia.
The study showed that this radiation is not produced by the Be star itself, as some hypotheses suggested, but by a magnetic white dwarf that orbits γ Cas. This conclusion puts an end to a mystery that has persisted for 50 years.
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The details were published last Tuesday, the 24th, in a scientific article in the journal Astronomy & Astrophysics.
In addition to solving the enigma about the X-rays from γ Cas, the research confirmed the existence of a class of binary systems predicted only theoretically.
Visible to the naked eye, γ Cas has been known since the 19th century as the first identified Be-type star. These stars are very massive, rotate rapidly, and eject material, forming disks around themselves.
Unusual behavior of the star has drawn attention since 1976
Since 1976, observations had revealed unusual behavior in γ Cas. The star emitted X-rays with an intensity about 40 times greater than that observed in similar stars.
Additionally, the plasma associated with these emissions exhibited temperatures exceeding 100 million degrees and extremely rapid variations.
These factors made γ Cas one of the most persistent cases in stellar astrophysics.
Various scenarios were proposed to explain the origin of the X-rays. Among them was the hypothesis of local magnetic reconnection between the surface of the Be star and its disk.
Other explanations suggested that the X-rays could be linked to a companion.
Among the possibilities raised were a star stripped of its outer layers, a neutron star, or an accreting white dwarf.
Even after decades of investigation and the identification of about 20 similar objects, called γ Cas analogs, none of these hypotheses had been conclusively proven.
High-precision observations changed the case
The answer came with the Resolve instrument, a high-precision microcalorimeter installed on the Japanese XRISM space telescope. The equipment is capable of analyzing X-ray spectra with unprecedented detail.
The team conducted three observation campaigns between December 2024 and June 2025. The measurements covered the entire orbital period of the binary system, estimated at approximately 203 days.
The data revealed decisive evidence. The spectral signatures of the hot plasma varied in speed over time, following the orbital motion of the companion star.
The spectra showed that the signatures of the high-temperature plasma changed speed between the three observations, following the orbital motion of the white dwarf, not that of the Be star.
The change was measured with high statistical reliability. Thus, the first direct evidence emerged that the ultra-hot plasma responsible for the X-rays is associated with the compact companion object, not with γ Cas itself.
The analysis of the width of the spectral lines, found at speeds of about 200 km/s, also allowed for the dismissal of the scenario of a non-magnetic white dwarf. The data indicated the presence of a significant magnetic field.
This magnetic field channels the accreting material. This result reinforced the identification of the magnetic white dwarf as the real origin of the phenomenon observed in the system.
Proposed model explains the origin of the X-rays
Based on the observations, the researchers proposed a model for the system. The Be star ejects material and forms a disk around it.
Part of this material is captured by the white dwarf, creating a second accretion disk. The magnetic field of the compact object directs this flow to its poles.
It is in this region that energy is released in the form of X-rays. Thus, the behavior of γ Cas now has a direct explanation supported by precise measurements.
The discovery resolves the case of γ Cas and confirms the existence of a population of binary systems composed of Be-type stars and accreting white dwarfs.
This class had been predicted for decades but had never been accurately identified.
Result also challenges theoretical models
The results, however, also challenge established theoretical models. The observations indicate that these systems correspond to about 10% of Be stars and are primarily linked to the more massive ones.
This picture contrasts with previous predictions, which pointed to a more numerous population composed of lower-mass stars. The discrepancy suggests the need for a revision of binary evolution models.
Among the points involved in this revision is the efficiency of mass transfer between the components. The conclusion aligns with that presented by several recent independent studies.
For the researchers, solving the case of γ Cas opens new avenues for research in the coming years. The discovery broadens the understanding of binary systems and their role in stellar evolution.
Researcher Yaël Nazé also highlighted the broader importance of the result. Understanding the evolution of binary systems is crucial, for example, for understanding gravitational waves, emitted by massive binary systems at the end of their lives.
With information from Galileu Magazine.
