Astronomers solve 50-year mystery of a naked-eye star’s extreme X-rays

What Makes Gamma Cassiopeiae So Unusual

γ Cassiopeiae was the first star classified as a Be-type star, identified in 1866 by Italian astronomer Angelo Secchi. These massive stars spin rapidly and regularly eject material into space. That material forms a disc around the star, which can 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 following two decades, space observatories found around twenty stars with similar behavior, now known as 'γ Cas analogues'. Astronomers at University of Liège played a major role in identifying more than half of these objects.

Competing Theories for the X-Ray Emission

"Several scenarios had been proposed to explain this emission," explains Yaël Nazé, an astronomer at ULiège. "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 did not match theoretical predictions. That left two possibilities: magnetic activity near the star or a nearby white dwarf pulling in material. Until recently, there was no clear way 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 is transforming high-energy astrophysics. Data were collected in December 2024, February 2025, and June 2025, covering the full 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 rather than that of the Be star," the researcher continues. "This shift was measured with high statistical reliability. It is, in fact, the first direct evidence the the ultra-hot plasma responsible for the X-rays is associated with the compact companion, and not with the Be star itself."

Evidence for a Magnetic White Dwarf

The measurements also provide insight into the nature 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. Instead, the results indicate a magnetic white dwarf, where the disc is cut off and the magnetic field directs incoming material toward its poles (see figure).

A New Class of Binary Stars Confirmed

These findings show that γ Cas and similar stars belong to a class 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. However, theoretical models had expected a larger population and suggested a stronger connection with 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 therefore opens up new avenues of research for the years to come! 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," concluded Yaël Nazé.