An active stellar corpse, caused by a distant star’s explosive death, was the likely source of repeated energetic flares observed over several months. Such a phenomenon had never been seen before and a team led by University of Cornell, including the Institute of Space Sciences (ICE-CSIC) and the Institute of Space Studies in Catalonia (IEEC), reports it in a new research published today in Nature.

The bright, brief flashes –as short as a few minutes in duration, and as powerful as the original explosion 100 days later– appeared in the aftermath of a rare type of stellar cataclysm that the researchers had set out to find, known as a luminous fast blue optical transient, or LFBOT. It has been officially labelled AT2022tsd and nicknamed “the Tasmanian devil”.

Since their discovery in 2018, astronomers have speculated about what might drive such extreme explosions, which are far brighter than the violent ends massive stars typically experience, but fade in days instead of weeks. The research team believes the previously unknown flare activity, which was studied by 15 telescopes around the world, confirms the engine must be a stellar corpse: a black hole or neutron star.

ICE-CSIC and IEEC researchers Lluís Galbany, Claudia Gutiérrez and Tomás E. Müller-Bravo contributed to the study as part of the ePESSTO+ (extended Public ESO Spectroscopic Survey of Transient Objects). The collaboration started in 2019 and focuses on exploring new transient populations.

Anna Y. Q. Ho, assistant professor of astronomy in the College of Arts and Sciences, is the first author of the study and helped characterise the ‘Tasmanian devil’ and the ensuing pulses of light seen roughly a billion light years from Earth. “We don’t think anything else can make these kinds of flares. This settles years of debate about what powers this type of explosion, and reveals an unusually direct method of studying the activity of stellar corpses.”, she says.

She wrote the software that flagged the event in September 2022, while sifting through a half-million changes, or transients, detected daily in an all-sky survey conducted by the California-based Zwicky Transient Facility. While routinely monitoring the fading explosion in December 2022, the team of researchers found that one of the analysed images picked up light, followed by an intensely bright spike in the middle frame that quickly vanished.

“No one really knew what to say,” Ho recalled. “We had never seen anything like that before – something so fast, and the brightness as strong as the original explosion months later – in any supernova or FBOT. We’d never seen that, period, in astronomy.”

“Finding and characterising LFBOTs is challenging due to their nature: they are objects that evolve very quickly. We lose important information about their explosion mechanisms and physics if we wait a few days to coordinate observations. Therefore, we need to trigger observations when you find a good candidate. Despite all our efforts, we have detected around a dozen LFBOTS, of which only six provide some clues about this new class of events. Of these, only one, the ‘Tasmanian devil’, shows these unprecedented flares”, says ICE-CSIC and IEEC researcher Claudia Gutiérrez.

To further investigate the abrupt rebrightening, the researchers engaged partners who contributed observations from more than a dozen other telescopes, including one equipped with a high-speed camera. The team combed through earlier data and worked to rule out other possible light sources. Their analysis ultimately confirmed at least 14 irregular light pulses over a 120-day period, likely only a fraction of the total number, Ho said.

Exploring the stellar evolution

Exactly what processes were at work – perhaps a black hole funnelling jets of stellar material outward at close to the speed of light – continues to be studied. The team hopes the research advances long-standing goals to map how stars’ properties in life may predict the way they’ll die, and the type of corpse they produce.

“In the case of LFBOTs, rapid rotation or a strong magnetic field are likely key components of their launching mechanisms”, Ho indicates. It’s also possible that they aren’t conventional supernovas at all, instead triggered by a star’s merger with a black hole.

“As the current sample of LFBOTs remains small, current and forthcoming high-cadence surveys, such as LS4, GOTO and BlackGem, will be essential for increasing their number and helping elucidate the progenitor scenario of this type of transients", points out ICE-CSIC and IEEC researcher Tomás E. Müller-Bravo.

“The unusual explosions promise to provide new insight into stellar life cycles typically only seen in snapshots of different stages – star, explosion, remnants – and not as part of a single system”, according to Ho. LFBOTs may present an opportunity to observe a star in the act of transitioning to its afterlife.

“It is a great time to study the transient Universe. Just five years ago it would have been impossible to detect such an event. Our supernova group at ICE-CSIC is putting an extra effort to be part of these international collaborations, and be able to be the first to detect, follow up and study extraordinary events like AT2022tsd”, adds ICE-CSIC and IEEC researcher Lluís Galbany.