Astronomers have detected exceptionally bright but brief flares of a type never seen before. Their conclusions are still tentative, but they suspect that we witnessed twitches from the corpse of a star that died months before. However, the exact nature of the cadaver in question remains uncertain.
In 2018 astronomers witnessed a strange type of exploding star they nicknamed the Cow (official designation AT2018cow). The discovery the Cow is not unique led to the creation of a class called Luminous Fast Blue Optical Transients (LFBOTs), of which we have since seen about one a year.
Once we have calculated the distance to each LFBOT we find their intrinsic brightness is similar to that of supernovae, but they fade in days, not weeks or months, suggesting a different process is involved. Just when it was thought we were getting an idea of what this process was, an LFBOT was discovered earlier this year that didn’t fit.
"The more we learn about LFBOTs, the more they surprise us," the European Space Agency’s Dr Ashley Chrimes said at the time of that discovery. That view is unlikely to change after analysis of AT2022tsd, which has won the nickname the Tasmanian Devil, and showed resilience we can only hope the threatened marsupials can match.
When first detected on September 7, AT2022tsd looked like a typical LFBOT a little over 4 billion light-years away. Then, exactly 100 days later, observers still tracking the event got an early Christmas present with a flare-up almost as bright as the original explosion.
“No one really knew what to say,” Cornell University’s Dr Anna Ho said in a statement. “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.”
Looking back through old images Ho and colleagues realized there had been another flare 26 days after the original burst.
The Tasmanian Devil had now moved far up the priority list for telescope observing time, and over 20 nights 13 telescopes detected 14 further flares, lasting from 10 minutes to 4.5 hours. Some flares were visible in only one part of the spectrum – such as one that could be seen in optical but not X-rays, despite both types of telescopes being focused on the same location at once. No gamma ray uptick was spotted.
“LFBOTs are already a kind of weird, exotic event, so this was even weirder,” Ho said. “We might be seeing a completely different channel for cosmic cataclysms.”
“To our knowledge, this phenomenon – minute-timescale optical flares at supernova-like luminosities, with order-of-magnitude amplitude variations, persisting for 100 days – has no precedent in the literature,” the team write, which is astronomer-speak for, “We’ve never seen anything like this before.”
Aside from events seen only in gamma rays, the closest counterparts have been far fainter and either much shorter or much longer.
The explanation the authors offer is that a compact object such as a black hole or neutron star is producing emissions at close to the speed of light. “We don’t think anything else can make these kinds of flares,” Ho said.
A plethora of explanations for LFBOTs have been offered previously, with the most popular being the collapse of a super-giant star, larger even than those that produce ordinary supernovas. Such an event would certainly be expected to leave a black hole behind, and it’s possible this former star is refusing to die quietly, instead forming an accretion disk that produces the flares we are seeing. However, plenty of other possibilities are also in the mix.
We might have seen this extended season of the Tasmanian Devil, and not other LFBOTs, because we are viewing it from a more face-on angle. Or maybe this one is truly different.
Such an oddity remains a scientific goldmine. “Because the corpse is not just sitting there, it’s active and doing things that we can detect,” Ho said. “We think these flares could be coming from one of these newly formed corpses, which gives us a way to study their properties when they’ve just been formed.”
The study is published in Nature.