Astronomers have finally identified the source of the unusual X-rays coming from the bright star Gamma-Cas. The culprit is an invisible companion star that is pulling material from its larger neighbor. This discovery puts an end to a mystery that has puzzled scientists for more than 50 years.
New high-resolution data from the X-ray Imaging Spectroscopy Mission (XRISM) showed that X-rays are tied to the orbit of a nearby white dwarf star. By tracking this movement, researchers were able to confirm the true origin of the emission. This finding is explained in a study led by Yael Nazeh from the University of Liège in Belgium.
“Many research groups have been working diligently for decades to unravel the mysteries of gamma Cas, and now, thanks to XRISM’s high-precision observations, we have finally achieved it,” says Yael.
A bright star with a long history of mysterious behavior
The star Gamma-Cas (γ-Cas) is visible to the naked eye and forms the central point of the familiar W-shaped constellation Cassiopeia, which can be seen on clear nights across Europe. Despite its brightness, questions have arisen since 1866, when Italian astronomer Angelo Secchi noticed something unusual in its light.
Instead of displaying dark hydrogen lines like the Sun, Gamma-Cas displayed bright hydrogen lines. This unexpected feature created a new category known as “Be” stars, a combination of “B” for hot blue-white stars and “e” for their distinctive emission lines.
It took scientists many years to realize that these emissions were coming from rotating disks of material thrown around by rapidly spinning stars. These disks can grow or disappear over time, causing changes in brightness and continue to fascinate amateur astronomers today.
Clues pointing to a hidden white dwarf family
As observations improved, astronomers detected subtle movements in Gamma-Cas that suggested the presence of a smaller companion star. Although the star can’t be seen directly, researchers believe it may be the remains of a white dwarf, a dense star with a mass similar to the Sun that was compressed to a size comparable to Earth.
A new mystery emerged in the 1970s, when it was discovered that gamma-Cas emits unusually strong X-rays. Further investigation revealed that these X-rays were coming from very hot plasma reaching temperatures of about 150 million degrees Celsius, making them much hotter and brighter than would be expected for such a star.
With the help of advanced X-ray observatories such as ESA’s XMM-Newton, NASA’s Chandra, and the German-led eROSITA, astronomers have identified about two dozen similar systems. These gamma-Cas type stars form a unique subgroup among Be stars due to their strong X-ray output.
XRISM data support accretion as an X-ray source
For years, scientists debated two main explanations. One idea suggests that magnetic interactions between the star and its surrounding disk produced the high-energy radiation. Another proposed that material from the disk falls onto a hidden companion star, producing the X-rays.
XRISM’s high-precision spectrometer Resolve finally provided the answer. Observations show that the hot plasma that produces the X-rays moves in step with the orbit of an invisible companion star. This confirms that the white dwarf is drawing material from gamma-Cas, producing X-rays as the material heats up.
“Our previous work with XMM-Newton really paved the way for XRISM, allowing us to eliminate a large number of theories and prove which of the last two competing theories is correct,” says Yaël. “We are very happy to finally have direct evidence that solves this mystery!”
New insights into binary star evolution
Identification of the gamma-Cas system as a pair of a Be star and an accreting white dwarf resolves a long-standing question about X-rays. At the same time, new questions arise about how these binary systems form and change over time.
Scientists once believed that such combinations were particularly common among low-mass stars. However, recent discoveries suggest that their frequency is lower than expected and that they are often associated with massive Be stars.
“We think the key is to understand how exactly the interaction occurs between two stars,” Yael says. “Now that we know the true nature of gamma-Cas, we can create specialized models for this class of star systems and update our understanding of binary evolution accordingly.”
“It’s incredible to see this mystery slowly unraveling over the years,” said Alice Borghese, an ESA researcher specializing in the field of high-energy astrophysics. “XMM-Newton laid a lot of groundwork in eliminating different theories about gamma Cas, and now, with the next generation of advanced instruments, XRISM has brought us to the finish line.”
“This impressive result highlights the strong collaboration between XRISM’s Japanese, European and American teams,” adds Matteo Guainazzi, XRISM project scientist at ESA. “This international team combines the technical and scientific expertise needed to solve the X-ray universe’s greatest mysteries and open new avenues of research.”
