Y. Nazé / ESA
Massive gamma-Cas star and white dwarf companion (artistic rendering)
γ Cas has intrigued astronomers since the 19th century. New study presents a model with a different explanation.
A Japanese space mission helped solve a astronomical mystery about 50 years old.
This “mystery” is the origin the intense emission of x-rays of the gamma star Cassiopeia – or γ Cas – visible to the naked eye in the constellation Cassiopeia.
which had new observations from the XRISM telescope, indicates that this radiation is not produced by the massive star itself (as was admitted for decades) but rather by a white dwarf that orbits it and that is capturing matter from the gas disk expelled by γ Cas.
γ Cas has intrigued astronomers since the 19th century. But the enigma took on a new dimension in the mid-1970s, when it was discovered that it emitted X-rays much more intense than would be normal for such a star.
Later studies showed that this emission was associated with extremely hot plasma, with temperatures in the order of 150 million degrees, and rapid variations that were difficult to explain.
Over time, other stars with similar behavior were identified, forming a small group of objects known as “Cas gamma analogues”remember .
For years, scientists were divided between two hypotheses main ones: one attributed the X-rays to magnetic interactions between the star and its disk; the other pointed to the fall of matter onto an invisible companion.
This XRISM mission, launched to observe the Universe in X-rays with high precision, finally made it possible to distinguish between the two scenarios.
According to the team led by astronomer Yaël Nazé, from the University of Liège, the signatures of the hot plasma follow the orbital movement of the companion: thus confirming that the source of the radiation is the white dwarf.
In other words, the new observations propose a clear model: the Be star ejects material that forms a disk around it; Some of this material is captured by the white dwarf, creating a second accretion disk. Then, the magnetic field of the compact object directs this flow towards its poles, where the energy is released in the form of X-rays.
