The star γ Cas, visible to the naked eye within the constellation Cassiopeia, has confused astronomers for many years. It produces X-rays way more intense and warmer than what scientists expect from a typical massive star. Latest observations using the Resolve instrument aboard Japan’s XRISM space telescope now link these emissions to a white dwarf orbiting the star. This discovery also confirms a long-predicted kind of binary system that had never been clearly identified. The findings, led by researchers on the University of Liège, were published in Astronomy & Astrophysics.
What Makes Gamma Cassiopeiae So Unusual
γ Cassiopeiae was the primary star classified as a Be-type star, identified in 1866 by Italian astronomer Angelo Secchi. These massive stars spin rapidly and usually eject material into space. That material forms a disc across the star, which might be detected through specific features in its optical spectrum.
In 1976, scientists realized that γ Cas emits X-rays about forty times stronger than similar stars. The plasma responsible reaches temperatures above 100 million degrees and changes rapidly. Over the next twenty years, space observatories found around twenty stars with similar behavior, now often called ‘γ Cas analogues’. Astronomers at University of Liège played a serious role in identifying greater than half of those objects.
Competing Theories for the X-Ray Emission
“Several scenarios had been proposed to clarify this emission,” explains Yaël Nazé, an astronomer at ULiège. “Certainly one of them involved local magnetic reconnection between the surface of the Be star and its disc. 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.”
Researchers had already ruled out stripped stars and neutron stars because observations didn’t match theoretical predictions. That left two possibilities: magnetic activity near the star or a close-by white dwarf pulling in material. Until recently, there was no clear solution to distinguish between them.
XRISM Data Tracks the Source of the X-Rays
To resolve the mystery, the team carried out a series of observations using Resolve, a high-precision microcalorimeter on board XRISM that’s transforming high-energy astrophysics. Data were collected in December 2024, February 2025, and June 2025, covering the total 203-day orbit of the system.
“The spectra revealed that the signatures of the high-temperature plasma change velocity between the three observations, following the orbital motion of the white dwarf relatively than that of the Be star,” the researcher continues. “This shift was measured with high statistical reliability. It’s, in truth, the primary direct evidence the the ultra-hot plasma answerable for the X-rays is related to the compact companion, and never with the Be star itself.”
Evidence for a Magnetic White Dwarf
The measurements also provide insight into the character of the white dwarf. The spectral features have a moderate width (of the order of 200 km/s), which rules out a non-magnetic white dwarf. In that scenario, material would fall inward through rapidly rotating inner regions of the disc, producing much broader signals. As an alternative, the outcomes indicate a magnetic white dwarf, where the disc is cut off and the magnetic field directs incoming material toward its poles (see figure).
A Latest Class of Binary Stars Confirmed
These findings show that γ Cas and similar stars belong to a category of Be + white dwarf binary systems that had long been predicted but never clearly observed. Researchers at ULiège also identified two key traits of this group. It mainly involves massive Be stars and represents about 10% of them. Nonetheless, theoretical models had expected a bigger population and suggested a stronger reference to lower-mass Be stars.
“This discrepancy suggests a revision of binary evolution models, particularly regarding the efficiency of mass transfer between components — a conclusion that aligns with that of several recent independent studies. Solving this mystery subsequently opens up latest avenues of research for the years to return! Understanding the evolution of binary systems is crucial for comprehending, for instance, gravitational waves, because it is indeed massive binaries that emit them at the tip of their lives,” concluded Yaël Nazé.