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MAGIC Telescope Array Observes Violent Explosion on “Vampire Star”

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Artist’s impression of material transfer from a red giant to a white dwarf. © https:​/​​/​ ​/​ MPP
Artist’s impression of material transfer from a red giant to a white dwarf. This is how RS Ophiuchi might also have looked prior to the nova outburst.

With the help of the MAGIC (Major Atmospheric Gamma Imaging Cherenkov) telescopes on La Palma in the Canary Islands, researchers from TU Dortmund University have detected very-high-energy gamma rays from a recurrent nova in the Milky Way. It is the first nova in which such high-energy rays have been evidenced. The event could deliver new insights into this type of explosion and the possible role they play in generating the mysterious high-energy cosmic rays that permeate our Milky Way. The researchers’ findings were published recently in the prestigious journal Nature Astronomy.

When a star dies, it first expands into a red giant star and then collapses into a stellar corpse, a white dwarf. This is composed of a very dense material: A teaspoon of it would weigh about a ton. Under certain circumstances, these stellar corpses can trigger enormous explosions a second time: If the white dwarf has a companion star that for its part passes into the red giant phase, the hydrogen from the giant’s expanded outer layers can succumb to the tremendous gravitational pull of the dense dwarf and accumulate on its surface. In the process, the “dead” star extracts gas from the active star and is therefore also known as a “vampire star”. In isolated cases, even nuclear explosions on the surface can occur in such systems, catapulting a large proportion of the hydrogen and the fusion products into space. As the explosion is extremely bright, the process is also referred to as “stella nova” (new star, in short “nova”). In some cases, the gas transfer repeats and thus also the nova outburst. The term for this is “recurrent nova”.

Very-high-energy gamma rays

One of these recurrent novae is RS Ophiuchi in our Milky Way, for which the next outburst had been expected last year. On 8 August 2021, telescopes were then indeed able to detect the light of an explosion. One day later, astronomers from the MAGIC consortium, an international alliance of around 160 researchers, aimed their telescopes at the eruption taking place. The telescopes are an array of two Imaging Atmospheric Cherenkov Telescopes (IACT) with a diameter of 17 meters. Thanks to the good observation conditions on La Palma and the unique sensitivity of the MAGIC array, it was possible to detect very-high-energy gamma rays during the nova, which could be traced back to proton acceleration. “The observation of celestial objects in the presence of such great energies opens unique windows into the extreme Universe. It allows us to study in detail the processes where particles in the Universe are accelerated to energies significantly higher than in terrestrial experiments,” explains Dr. Dominik Elsässer from the Department of Physics and member of the MAGIC collaboration’s executive board.

The MAGIC telescope array during observation of the nova outburst. © Urs Leutenegger
The MAGIC telescope array during observation of the nova outburst of RS Ophiuchi in the night that the very-high-energy gamma rays were detected (11 August 2021).

Taken in isolation, nova outbursts are less energetic than their sisters – supernovae, in which a whole star is torn apart in an explosion – but they occur much more often. The results suggest that although most of the high-energy cosmic rays that permeate the Milky Way probably emanate from other sources, novae are apparently surprisingly efficient at generating local regions with an overdensity of cosmic rays in their surrounding environment. To fully understand such explosive events, further observations are needed. The research groups at TU Dortmund University are involved here, in particular with detector simulations and the development of intelligent analysis software. In addition, since January 2022 researchers from TU Dortmund University, Ruhr-Universität Bochum (RUB) and the University of Wuppertal have dedicated themselves, in the framework of Collaborative Research Center (CRC) 1491, to understanding the processes taking place in the cosmic interaction of various forms of matter. “It’s above all interdisciplinary cooperation between particle physics, astrophysics, plasma physics and data science that first makes fundamental breakthroughs possible,” says Professor Wolfgang Rhode, professor for astroparticle physics at TU Dortmund University and co-spokesperson of CRC 1491.

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