Space and Physics
PUBLISHED
From the moment that astronomer Angelo Secchi identified γ Cassiopeia (gamma-Cas) as the first Be star in 1866, the object has been nothing but trouble. A massive, quickly rotating star that regularly ejects material, forming a disk around it. On top of that, 110 years later, astronomers discovered that it has an X-ray luminosity 40 times brighter than comparable stars. Now, 50 years later, there is finally an answer to what is causing that.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.“Several scenarios had been proposed to explain this emission,” lead author Yaël Nazé, an astronomer at the University of Liège, said in a statement. “One of them involved local magnetic reconnection between the surface of the Be star and its disc. Others suggested X-rays to be linked to a companion, whether a star stripped of its outer layers, a neutron star, or an accreting white dwarf.”
Using data from the Japanese X-Ray Imaging and Spectroscopy Mission (XRISM), the team was able to resolve that the X-ray emission is linked to the orbital motion of a companion white dwarf. A white dwarf is the end of life for stars not massive enough to go supernova. Their outer layers are blown away, and their cores contract into dense and hot objects.
“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,” added Nazé in a different statement.
The XRISM observations built on many years of observations from other telescopes, such as the European Space Agency’s XMM-Newton, NASA’s Chandra, and the German-led eROSITA. Previous work allowed us to eliminate the alternative hypotheses, but the latest observations were needed to confirm that it was indeed a white dwarf that was causing the emission.
The plasma moves from the main star to the white dwarf, which is actively consuming it. This makes this material extremely hot, reaching 150 million kelvin – so hot that it shines in X-rays. Such a pairing was once thought common among low-mass stars, but it seems that these instead are in high-mass Be stars.
“We think the key is in understanding how exactly the interactions take place between the two stars,” explained Nazé. “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.”
The study is published in the journal Astronomy & Astrophysics.
