What do a cow, a koala, a camel, a Tasmanian devil and now a finch all have in common?
It’s not the plot of a new movie in the “Madagascar” series. They are nicknames given to a weird class of space explosions scientists can’t explain.
“We’ve been naming these things after animals just for fun,” said Daniel Perley, an astronomer at Liverpool John Moores University in England.
The latest, the Finch, was first spotted on April 10 using the Palomar Observatory in California. After a few weeks, Dr. Perley and his colleagues confirmed that the event matched its companions. A new animal had been found outside of a galaxy for the first time, making it the most intriguing detection yet.
So, what are these things? They are more technically called luminous fast blue optical transients, or LFBOTs. These space explosions are much brighter than supernovas (which occur when stars explode), hence the “luminous” designation. They brighten quickly — hence “fast” — and are extremely hot, reaching 70,000 degrees Fahrenheit, and thus emitting “blue” light.
“Typically supernovae brighten and fade over weeks to months,” said Deanne Coppejans, an astronomer at Warwick University in England. “These LFBOTs brighten in just three to four days and fade on much faster time scales.”
The first to be found was the Cow in 2018, its name deriving from the sequence of letters and number it was automatically assigned — AT2018cow. Among the half dozen since discovered are ZTF18abvkwla (the Koala) and AT2022tsd (the Tasmanian devil).
AT2023fhn (the Finch) is the latest, given its name in a paper led by Ashley Chrimes, an astrophysicist from Radboud University in the Netherlands — although Dr. Perley and colleagues had been referring to it as “the Fawn” and may continue to do so. “We’re a little bit annoyed that they decided it was up to them to name the object,” he said.
Dr. Chrimes’s paper, submitted to The Monthly Notices of the Royal Astronomical Society for peer review, highlights the most unusual feature of the Finch, namely that it was found outside any galaxy, seemingly exploding in intergalactic space near two potential host galaxies about three billion light-years from our own.
“It’s about three to four times outside the radius of the galaxies,” Dr. Chrimes said. “At that distance you don’t expect to have many, or any, stars.”
That could help astronomers clarify what an LFBOTs is. At the moment, there are a few leading ideas. The most promising one is that it is a giant star, about 20 times the mass of our sun, that has undergone a failed supernova as a black hole forms at its core. If the star is massive enough, it might collapse into the black hole, rather than exploding back out as a supernova. That produces powerful jets that shoot outward and are detected as LFBOTs.
“That’s the model I’ve tended to favor over the past few years,” Dr. Perley said, “but I wouldn’t say I’m willing to definitively say that’s the one.”
The Finch could pose problems for that model. “It’s so far outside of the two nearby galaxies,” Dr. Chrimes said. “A massive star shouldn’t have been able to get there,” unless there is an unseen cluster of stars in its vicinity.
Another possibility is that LFBOTs are tidal disruption events, where a black hole is eating material from a companion star, shining brightly in the process. “But that also has a bit of a problem, because you don’t expect to have massive black holes outside galaxies,” Dr. Chrimes said.
LFBOTs could also be caused by the merger of two neutron stars, the remnant cores of dead massive stars. That process can take billions of years — which might “give them time to migrate far away from their galaxies” as the Finch did, Dr. Chrimes said.
Anna Y.Q. Ho, an astronomer at Cornell University who has also closely studied the Finch (or the Fawn), isn’t ready to jump to any conclusions. “It’s hard to make a statement from just one object,” she said. “You can’t really rule anything out.”
Astronomers are finding about one or two new LFBOTs every year. But upcoming telescopes, such as an Israeli-led ULTRASAT space telescope mission, set to launch in 2026, could greatly increase that number.
“It could find 10 or even 100 per year,” Dr. Ho said. “It will really dominate the rate of discovery.”
Finding more LFBOTs outside galaxies could provide hints to their origin. And as the discoveries keep trickling in, astronomers intend to keep going with their naming system — which, aside from being a bit of fun, is useful to better coordinate the findings. “People are much better at remembering properties of events and classifying them when there’s a more recognizable name,” Dr. Ho said.
“There aren’t that many of them yet,” she said. “So we haven’t run out of animals.”