The macroscopic star γ Cas in the constellation Cassiopeia has confounded astronomers for decades. It produces X-rays that are much more powerful and hot than what scientists would expect from a typical massive star. New observations using the Resolve instrument aboard Japan’s XRISM space telescope now link these emissions to white dwarfs orbiting the star. The discovery also confirms a long-predicted type of binary star system that has not been clearly identified. The results of the study, led by researchers at the University of Liège, astronomy and astrophysics.
What is so rare about gamma casiopeias?
Gamma Cassiopeiae was the first star to be classified as a Be star by Italian astronomer Angelo Secchi in 1866. These giant stars rotate at high speeds and periodically eject material into space. The material forms a disk around the star and can be detected through specific features in its optical spectrum.
In 1976, scientists noticed that γ Cas emitted X-rays about 40 times more intense than similar stars. The plasma responsible can reach temperatures in excess of 100 million degrees Celsius and change rapidly. Over the next 20 years, space observatories discovered about 20 stars with similar behavior, now known as “γ Cas analogues.” Astronomers at the University of Liege played a major role in identifying more than half of these objects.
Competing theories about X-ray radiation
“Several scenarios had been proposed to explain this emission,” explains Uriege astronomer Yael Nazeh. “One of them concerned local magnetic reconnection between the Be star’s surface and its disk. Others suggested that the X-rays become associated with the companion star, whether it is a stripped star, a neutron star, or an accreting white dwarf.”
The researchers had already ruled out exfoliated or neutron stars because their observations did not match theoretical predictions. That leaves two possibilities: magnetic activity near the star or a nearby white dwarf star is pulling the material. Until recently, there was no clear way to tell them apart.
XRISM data tracks the source of X-rays
To solve this mystery, the team performed a series of observations using Resolve, a high-precision microcalorimeter aboard XRISM that is transforming high-energy astrophysics. Data were collected in December 2024, February 2025, and June 2025, covering the system’s entire 203-day orbit.
“The spectra reveal that the hot plasma signature changes velocity between the three observations, following the orbital motion of the white dwarf rather than the orbital motion of the Be star,” the researchers continued. “This change was measured with high statistical confidence. In fact, this is the first direct evidence that the ultrahot plasma responsible for the X-rays is associated with a compact companion star rather than the Be star itself.”
Evidence for magnetic white dwarfs
The measurements also provide insight into the nature of white dwarfs. The spectral features have a moderate width (around 200 km/s) and exclude non-magnetic white dwarfs. In that scenario, material would fall inward through the rapidly rotating inner region of the disk, producing a broader signal. Instead, the results show a magnetic white dwarf, in which the disk separates and the magnetic field directs incoming material toward its poles (see diagram).
New class of binary stars confirmed
These findings indicate that stars similar to γ Cas belong to the long predicted but never clearly observed class of Be + white dwarf binary systems. The Uriege researchers also identified two important characteristics of this group. This mainly involves large Be stars, accounting for about 10% of them. However, theoretical models predicted a larger population and suggested a stronger relationship with low-mass Be stars.
“This discrepancy suggests a revision of the binary evolution model, especially with regard to the efficiency of mass transfer between components. This conclusion is consistent with that of several recent independent studies. Solving this mystery will therefore be a major challenge in the coming years. “This opens up research avenues! Understanding the evolution of binary star systems is crucial for understanding gravitational waves, for example, because it is actually the massive binaries that emit gravitational waves at the end of their lives,” concluded Yael Nadze.
