After more than five decades, astronomers have finally solved the mystery behind the unusual X-ray emissions from gamma-Cas (γ-Cas), a bright star visible in the constellation Cassiopeia. The breakthrough comes thanks to high-resolution observations from the X-Ray Imaging and Spectroscopy Mission (XRISM), a collaboration between JAXA, NASA, and ESA.
The star’s peculiarities date back to 1866, when Italian astronomer Angelo Secchi noted a strange bright hydrogen signature, defining a new class of “Be” stars. By the mid-1970s, gamma-Cas was found to shine in unexpected high-energy X-rays, producing plasma heated to 150 million degrees—far brighter than typical for such massive stars.
For years, the explanation boiled down to two competing theories: either magnetic fields interacting with the star’s surrounding disk were generating the X-rays, or material from the Be star was being consumed by an unseen white dwarf companion.
Led by Yaël Nazé of the University of Liège, researchers used XRISM’s Resolve spectrometer to observe gamma-Cas with unprecedented precision. The instrument revealed that the signatures of the hot plasma follow the orbital motion of the invisible companion, confirming that a white dwarf is accreting material from the Be star. This accretion process generates the intense X-ray glow.
“There has been an intense effort to solve the mystery of gamma-Cas across many research groups for many decades. And now, thanks to the high-precision observations of XRISM, we have finally done it,” said Nazé.
The findings close a chapter on a longstanding astronomical puzzle while opening new questions. Such binary systems—a massive Be star paired with a compact white dwarf—were long thought to be common but appear rarer than expected, primarily forming around high-mass stars.
“Now that we know the true nature of gamma-Cas, we can create models specifically for this class of stellar systems, and update our understanding of binary evolution accordingly,” added Nazé.
ESA Research Fellow Alice Borghese noted the collaborative effort behind the result, with previous missions like XMM-Newton laying critical groundwork. ESA’s XRISM Project Scientist, Matteo Guainazzi, emphasized the mission’s international success, stating that the team combines the expertise needed to solve the Universe’s biggest X-ray mysteries.
