New high-resolution data from the X-Ray Imaging and Spectroscopy Mission (XRISM) showed that the X-rays are tied to the orbit of a nearby white dwarf star. By tracking this motion, researchers were able to confirm the true origin of the emissions. The findings are described in a study led by Yaël Nazé of the University of Liège, Belgium.

"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," says Yaël.

A Bright Star With a Long History of Puzzling Behavior

The star gamma-Cas (γ-Cas) can be seen with the naked eye and forms the central point of the familiar W-shaped constellation Cassiopeia, visible on clear nights across Europe. Despite its brightness, it has raised questions since 1866, when Italian astronomer Angelo Secchi noticed something unusual in its light.

Instead of showing a dark hydrogen line like the Sun, gamma-Cas displayed a bright one. This unexpected feature led to the creation of a new category known as 'Be' stars, combining the "B" for hot, blue-white stars with the "e" for their distinctive emission lines.

It took many years for scientists to understand that these emissions come from a spinning disc of material thrown off by the rapidly rotating star. These discs can grow and fade over time, causing changes in brightness that continue to attract amateur astronomers today.

Clues Point to a Hidden White Dwarf Companion

As observations improved, astronomers detected subtle movements in gamma-Cas that hinted at the presence of a smaller companion. Although it cannot be seen directly, researchers suspected it might be a white dwarf, a dense stellar remnant with a mass similar to the Sun but compressed into a size comparable to Earth.

A new puzzle appeared in the 1970s when gamma-Cas was found to emit unusually strong X-rays. Further investigation showed that these X-rays came from extremely hot plasma reaching temperatures of about 150 million degrees, far hotter and brighter than expected for a star like this.

With the help of advanced X-ray observatories such as ESA's XMM-Newton, NASA's Chandra, and the Germany-led eROSITA, astronomers identified about two dozen similar systems. These gamma-Cas-type stars form a unique subgroup among Be stars because of their intense X-ray output.

XRISM Data Confirms Accretion as the Source of X-Rays

For years, scientists debated two main explanations. One idea suggested that magnetic interactions between the star and its surrounding disc created the high-energy emissions. The other proposed that material from the disc was falling onto the hidden companion and producing X-rays.

XRISM's highly precise spectrometer Resolve finally provided the answer. Observations showed that the hot plasma producing the X-rays moves in step with the orbit of the unseen companion. This confirms that the white dwarf is pulling in matter from gamma-Cas and generating X-rays as the material heats up.

"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!"

New Insights Into Binary Star Evolution

Identifying gamma-Cas systems as pairs of Be stars and accreting white dwarfs answers the long-standing question about their X-rays. At the same time, it raises new questions about how these binary systems form and change over time.

Scientists once believed such pairings would be common, especially among lower-mass stars. However, recent findings suggest they are less frequent than expected and are more often associated with massive 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 specializing 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."