New observations from the Japanese XRISM Space Telescope have shown that Cassiopeia's intense, hot gamma-ray bursts do not originate from the star itself, but from an absorbing white dwarf hidden in the system—confirming for the first time a binary type that was proposed decades ago.
Astronomers have been puzzled for more than fifty years about the star Gamma Cassiopeiae, easily visible in the night sky in the constellation Cassiopeia. It produces X-ray radiation with energies and temperatures far above those expected from a typical massive star. New observations using the Resolve instrument on Japan's Charismatic Space Telescope have now revealed that the source of this unusual emission is a white dwarf orbiting the star. The finding also confirms a long-standing theory that a type of binary system exists that has never been clearly identified.
What is special about Kokhavi? B as in Gamma in Cassiopeia
Gamma Cassiopeiae was the first B-type star to be identified, a classification introduced in 1866 by Italian astronomer Angelo Sci. These stars are massive and rotate very rapidly, and periodically eject material into space. This material forms a surrounding disk, which can be detected by unique features in the star's optical spectrum.
In 1976, scientists discovered that Gamma Cassiopeiae emits X-rays about forty times more intensely than similarly massive stars. The plasma that produces these emissions reaches temperatures of over 100 million degrees and is rapidly changing. Over the next two decades, space-based observations identified about twenty stars with similar behavior, forming a group called the Gamma Cassiopeiae parallels.
Competing theories about the source of X-ray radiation
"Several scenarios have been proposed to explain this emission," explains Yael Naza, an astronomer at the University of Liège. "One of them involves local magnetic fusion between the surface of the B star and its disk. Other theories have suggested that the X-ray emission is associated with a companion, whether it is a star that has shed its outer layers, a neutron star, or an accreting white dwarf."
Previous studies have already ruled out "peeled" stars and neutron stars because the observations did not match theoretical expectations. Two possibilities remain: magnetic interactions involving the star itself or an accreted white dwarf companion. Until now, there has been no clear evidence to decide between the two.
Charisma's observations reveal the true source
To solve the mystery, the team performed three dedicated observations using Resolve, a highly accurate microcalorimeter on the CHRISMS that is revolutionizing the study of high-energy astrophysics. The observations covered the entire 203-day orbit of the system.
"The spectra revealed that the signatures of the high-temperature plasma change rapidly between the three observations, following the orbital motion of the white dwarf and not that of the B star," the researcher continued. "This change is measured with high statistical reliability. This is, in fact, the first direct evidence that the very hot plasma responsible for the X-ray emission is associated with the compact companion and not with the B star itself."
For the scientific article: DOI: 10.1051/0004-6361/202558284
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