doi.org/10.1051/0004-6361/202558284
Credibility: 989
#Gamma Cassiopeiae
The star Gamma Cassiopeiae, known as – Cas, is one of the brightest in the night sky and can be seen with the naked eye in the constellation Cassiopeia, forming part of the famous “W”
For over 50 years, since 1976, it has intrigued astronomers by emitting extremely intense X-rays-about 40 times stronger than those of similar massive stars.
These X-rays are generated by a plasma at temperatures exceeding 100 million degrees and vary rapidly, which did not fit the expected behavior of a common star.
Gamma Cassiopeiae is a Be-type star: a massive star that rotates very rapidly and periodically ejects material, forming a disk around it.
This disk is visible in the optical spectrum and was what first caught attention in 1866, when it became the first Be star identified.
But the enigma lay in the X-rays: where exactly did they come from? Some theories suggested they were produced within the star itself or its disk through magnetic processes.
Others pointed to a possible unseen companion, such as a white dwarf, a neutron star, or something similar.
For decades, scientists observed the system with different space telescopes.
Previous observations had already ruled out some hypotheses, such as the presence of a neutron star or a “naked” star.
The question remained whether their origin was within the star Be itself or in a compact companion.
Now, the mystery has finally been solved thanks to precise observations made by the Japanese XRISM telescope, using the Resolve instrument-a high-resolution microcalorimeter that allows for detailed analysis of X-rays.
An observation campaign conducted in December 2024, February, and June 2025 tracked the entire orbital period of the system, which lasts 203 days.
New data revealed that the X-rays do not originate from the primary star or its disk, but from a small, dense companion: a magnetic white dwarf orbiting Gamma Cassiopeiae.
The spectrum showed that the velocities of the hot plasma changed according to the orbital motion of the white dwarf, not the Be star.
This is the first direct evidence that the ultra-hot plasma is associated with the compact companion.
The white dwarf possesses a magnetic field strong enough to disrupt the disk of material flowing from the larger star.
This material is directed towards the poles of the white dwarf, where it falls and heats up violently, emitting the observed X-rays.
Some of this radiation is also reflected by the surface of the white dwarf.
The moderate broadening of the spectral lines (about 200 km/s) confirms this magnetic accretion scenario.
This discovery confirms the existence of a class of binary systems formed by a Be star and a magnetic white dwarf, something that theoretical models predicted, but which had never been observed so clearly.
It is estimated that about 10% of massive Be stars may belong to this type of system.
The research, led by Yaël Nazé of the University of Liège, along with other scientists such as Masahiro Tsujimoto, Gregor Rauw, and Sean J.
Gunderson, was published on March 24, 2026, in the journal “Astronomy & Astrophysics”.
Besides solving a half-century-old enigma, the result forces astronomers to revise models of binary star evolution, especially regarding mass transfer between stars, and opens new doors for studying gravitational waves and other interactions in similar systems.
As Yaël Nazé summarized, solving this mystery opens avenues for new research in the coming years.
What seemed like a secret hidden by a bright star now reveals the complex dance of two invisibly linked stars.
Published in 03/26/2026 00h07
Text adapted by AI (Grok) and translated via Google API in the English version. Images from public image libraries or credits in the caption. Information about DOI, author and institution can be found in the body of the article.
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