A stellar corpse shows signs of life through unprecedented flares

• An active stellar corpse, possibly a black hole or neutron star, was the likely source of repeated energetic flares never seen before
• The explosions are far brighter than the violent ends massive stars typically experience, but fade in days instead of weeks
• IEEC researchers at the Institute of Space Sciences (ICE-CSIC) participate in this new study published in Nature

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 researchers from the Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) at the Institute of Space Sciences (ICE-CSIC), 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.

Lluís Galbany, Claudia Gutiérrez and Tomás E. Müller-Bravo, IEEC researchers at the ICE-CSIC, 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,” explains Claudia Gutiérrez. “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”, she adds.

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. LFBOTs may present an opportunity to observe a star in the act of transitioning to its afterlife.

“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 Tomás E. Müller-Bravo.

“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”, concludes Lluís Galbany.

Press release prepared in collaboration with the Institute of Space Sciences (ICE-CSIC).

More information

This research is presented in a paper entitled “Minutes-duration Optical Flares with Supernova Luminosities”, by Anna Y. Q. Ho et al., which appeared in the journal Nature on 15 November 2023.

Links

• IEEC
• ICE-CSIC
• ePESSTO+

Contacts

About the IEEC

The Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) promotes and coordinates space research and technology development in Catalonia for the benefit of society. IEEC fosters collaborations both locally and worldwide and is an efficient agent of knowledge, innovation and technology transfer. As a result of more than 25 years of high-quality research, done in collaboration with major international organisations, IEEC ranks among the best international research centres, focusing on areas such as: astrophysics, cosmology, planetary science, and Earth Observation. IEEC’s engineering division develops instrumentation for ground- and space-based projects, and has extensive experience in working with private or public organisations from the aerospace and other innovation sectors.

The IEEC is a non-profit public sector foundation that was established in February 1996. It has a Board of Trustees composed of the Generalitat de Catalunya, Universitat de Barcelona (UB), Universitat Autònoma de Barcelona (UAB), Universitat Politècnica de Catalunya · BarcelonaTech (UPC), and the Spanish Research Council (CSIC). The IEEC is also a CERCA centre.