Radio signal reveals supernova origin

In the latest issue of the journal Nature, astronomers from Stockholm University reveal the origin of a thermonuclear supernova explosion. Strong emission lines of helium and the first detection of such a supernova in radio waves show that the exploding white dwarf star had a helium-rich companion.

Supernovae of Type Ia are important for astronomers since they are used to measure the expansion of the Universe. However, the origin of these explosions has remained an open question. While it is established that the explosion is that of a compact white dwarf star somehow accreting too much matter from a companion star, the exact process and the nature of the progenitor is not known. The new discovery of supernova SN 2020eyj established that the companion star was a helium star that had lost much of its material just prior to the explosion of the white dwarf.

"Once we saw the signatures of strong interaction with the material from the companion we tried to also detect it in radio emission," explains Erik Kool, post-doc at the Department of Astronomy at Stockholm university and lead author of the paper. "The detection in radio is the first one of a Type Ia supernova -- something astronomers have tried to do for decades."

Supernova 2020eyj was discovered by the Zwicky Transient Facility camera on Palomar mountain, where the Oskar Klein Centre at Stockholm University are members.

"The Nordic Optical telescope on La Palma was fundamental for following up this supernova," says Professor Jesper Sollerman at the Department of Astronomy and co-author of the paper. "As were spectra from the large Keck telescope on Hawai'i that immediately revealed the very unusual helium-dominated material around the exploded star."

"This is clearly a very unusual Type Ia supernova, but still related to the ones we use to measure the expansion of the universe," adds Joel Johansson from the Department of Physics.

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Mysteriously bright flash is a black hole jet pointing straight toward Earth, astronomers say

Astronomers have determined the source of an incredibly bright X-ray, optical and radio signal appearing from halfway across the Universe.

The signal, named AT 2022cmc, was discovered earlier this year by the Zwicky Transient Facility in California. Findings published today in Nature Astronomy, suggest that it is likely from a jet of matter, streaking out from a supermassive black hole at close to the speed of light.

The team, including researchers from MIT and the University of Birmingham, believe the jet is the product of a black hole that suddenly began devouring a nearby star, releasing a huge amount of energy in the process. Their findings could shed new light on how supermassive black holes feed and grow.

Astronomers have observed other such "tidal disruption events," or TDEs, in which a passing star is torn apart by a black hole's tidal forces. However AT 2022cmc is brighter than any TDE discovered to date, and is also the farthest TDE ever detected, at some 8.5 billion light years away.

The team measured the distance to the AT 2022cmc using the European Southern Observatory's Very Large Telescope, in Chile.

Dr Matt Nicholl, associate professor at the University of Birmingham, said: "Our spectrum told us that the source was hot: around 30,000 degrees, which is typical for a TDE. But we also saw some absorption of light by the galaxy where this event occurred. These absorption lines were highly shifted towards redder wavelengths, telling us that this galaxy was much further away than we expected!"

How could such a distant event appear so bright in our sky? The team says the black hole's jet may be pointing directly toward Earth, making the signal appear brighter than if the jet were pointing in any other direction. The effect is "Doppler boosting," and is similar to the amped-up sound of a passing siren.

AT 2022cmc is the fourth Doppler-boosted TDE ever detected and the first such event that has been observed since 2011. It is also the first boosted TDE discovered using an optical sky survey. As more powerful telescopes start up in the coming years, they will reveal more TDEs, which can shed light on how supermassive black holes grow and shape the galaxies around them.

Following AT 2022cmc's initial discovery, the team focused in on the signal using the Neutron star Interior Composition ExploreR (NICER), an X-ray telescope that operates aboard the International Space Station.

"Things looked pretty normal the first three days," recalls Dheeraj "DJ" Pasham, who was first author on the study. "Then we looked at it with an X-ray telescope, and what we found was, the source was 100 times more powerful than the most powerful gamma-ray burst afterglow."

Typically, such bright flashes in the sky are gamma-ray bursts -- extreme jets of X-ray emissions that spew from the collapse of massive stars.

Dr Benjamin Gompertz, assistant professor at the University of Birmingham, led the gamma-ray burst comparison analysis. "Gamma-ray bursts are the usual suspects for events like this." he said. "However, as bright as they are, there is only so much light a collapsing star can produce. Because AT 2022cmc was so bright and lasted so long, we knew that something truly gargantuan must be powering it -- a supermassive black hole."

The extreme X-ray activity is believed to be powered by an "extreme accretion episode" when the shredded star creates a whirlpool of debris as it falls into the black hole. Indeed, the team found that AT 2022cmc's X-ray luminosity was comparable to, though brighter than, three previously detected TDEs.

"It's probably swallowing the star at the rate of half the mass of the sun per year," Pasham estimates. "A lot of this tidal disruption happens early on, and we were able to catch this event right at the beginning, within one week of the black hole starting to feed on the star."

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