XRISM solves famous star’s 50-year mystery

24/03/2026
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An invisible companion consuming material from the naked-eye star gamma-Cas has been revealed as the culprit for curious X-rays coming from the stellar system. This closes the case on a mystery that has puzzled astronomers for more than fifty years. 

A mystery steeped in history

The star gamma-Cas (γ-Cas) is visible to Europeans every cloudless night. It makes up the central ‘point’ of the distinctive ‘W’-shaped constellation Cassiopeia. 

Despite its prominence in the night sky, it has been shrouded in mystery since 1866 when Italian astronomer Angelo Secchi noticed something odd in its light signature. Its hydrogen ‘fingerprint’ was bright, whereas in stars like our own Sun this normally shows up as a dark line.

This weird feature inaugurated a new class of stars, called ’Be’ stars, merging the ‘B’ associated with hot blue-white massive stars with the ‘e’ from the peculiar hydrogen emission.

It took several decades before astronomers understood that these emissions were coming from a rotating disc of matter ejected by the fast-spinning star. Such discs can build and disperse over time, resulting in variations in the star’s brightness. This makes it a popular target for amateur astronomers still today.

As telescope observations became more refined, monitoring gamma-Cas’s motion was possible, revealing that it must have a low-mass companion star. Since the companion remains invisible to spot directly with telescopes, astronomers think it might be a white dwarf – a compact object with the mass of the Sun but the size of Earth.

Then, in the mid-1970s, a new mystery emerged: gamma-Cas was discovered to shine in unusual high-energy X-rays. Further studies found the origin of this X-ray glow to be mostly coming from extremely hot 150-million-degree plasma, shining with a brightness some 40 times greater than typically expected for such massive stars.

With the dawn of X-ray space telescopes including ESA’s XMM-Newton, NASA’s Chandra and the Germany-led eROSITA, astronomers have found around two dozen gamma-Cas-type stars with similar, unusual X-ray emission, making them a special group among Be stars in general.

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.”

Notes for editors

‘Orbital motion detected in γ Cas Fe K emission lines’ by Y. Nazé et al is published in Astronomy & Astrophysics.

XRISM (pronounced krizz-em) was launched on 7 September 2023. It is a mission led by the Japan Aerospace Exploration Agency (JAXA) in partnership with NASA and ESA. It carries two instruments: an X-ray calorimeter called Resolve capable of measuring the energy of individual X-ray photons to produce a spectrum at unprecedented level of ‘energy resolution’ (the capability of an instrument to distinguish the X-ray ‘colours’), and a large field-of-view X-ray CCD camera to image the surrounding field called Xtend.

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