In the last issue of the magazine Nature Stockholm University astronomers reveal the origin of the thermonuclear supernova explosion. Strong helium emission lines and the first radio detection of such a supernova indicate that the exploding white dwarf is a helium-rich companion.

Type Ia supernovae are important to astronomers as they are used to measure the expansion of the universe. However, the question of the origin of these eruptions remains open. While the explosion has been determined to be the explosion of a compact white dwarf star that somehow picked up a lot of matter from a companion star, the exact process and nature of its progenitor is unknown. The new discovery of supernova SN 2020eyj has revealed that the companion star is a helium star that lost most of its material just before the white dwarf exploded.

“After seeing the signatures of a strong interaction with the companion material, we tried to detect it in the radio emission as well,” explains Erik Kuhl, a postdoctoral researcher in the Department of Astronomy at Stockholm University and lead author of the paper. “Radio detection is the first Type Ia supernova — something astronomers have been trying to do for decades.”

Supernova 2020eyj was detected by the Zwicky Transitional Facility on Palomar Mountain, of which the Oscar Klein Center at Stockholm University is also a member.

“The Scandinavian Optical Telescope at La Palma was fundamental to the observation of this supernova,” says Professor Jesper Sollerman, co-author of the paper and from the Department of Astronomy. “Like the spectrum from the Large Keck Telescope in Hawaii, which instantly detects very unusual helium-heavy material around the exploding star.”

“This is clearly a very unusual type Ia supernova, but it still relates to the way we use it to measure the expansion of the universe,” adds Joel Johansson from the Department of Physics.

“While normal Type Ia supernovas always explode with the same brightness, this supernova tells us there are many different ways a white dwarf star can explode,” he adds.