The final two theories

Over the years, the explanation for the high-energy X-rays boiled down to two competing theories. Could the star’s local magnetic fields be interacting with that of its surrounding disc, producing the hot material? Or, are X-rays generated by the Be star’s disc material falling onto the white dwarf companion?

Finally, an instrument exists with high enough precision to solve the mystery: XRISM’s high-resolution spectrometer Resolve. In a dedicated observation campaign XRISM revealed that the signatures of the hot plasma follow the orbital motion of the otherwise invisible companion star. In other words, the white dwarf companion consumes material from gamma-Cas, emitting X-rays as it does so.

“The previous work using XMM-Newton really cleared the way for XRISM, enabling us to eliminate numerous theories and prove which of the last two competing theories was correct,” says Yaël. “It’s extremely satisfying to have direct evidence to solve this mystery at long last!”

Understanding that gamma-Cas objects are Be type stars paired with a white dwarf that’s accreting material, solves the X-ray mystery. But it also opens up another curiosity in terms of how the wider population of this type of binary systems forms and evolves.

Such pairs were long expected to be common, mainly among low‑mass stars. However, new research shows they are rarer than predicted and instead tend to occur in high‑mass Be stars.

“We think the key is in understanding how exactly the interactions take place between the two stars,” says Yaël. “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.”

“It’s incredible to see how this mystery has slowly unfolded over the years,” says Alice Borghese, an ESA Research Fellow specialising in the field of high-energy astrophysics. “XMM-Newton did so much of the groundwork in ruling out various theories about gamma-Cas. And now with the next generation of advanced instrumentation, XRISM has brought us over the finish line.”

“This wonderful result underlines the strong collaboration between XRISM’s Japanese, European and American teams,” adds Matteo Guainazzi, ESA’s XRISM Project Scientist. “This international team combines the technical and scientific expertise needed to solve the X-ray Universe’s biggest mysteries and open new avenues for research.”