The first explosion, dubbed AT2018cow and nicknamed "the Cow," appeared in a galaxy 200 million light-years from Earth. It was detected by the Asteroid Terrestrial-impact Last Alert System (Atlas), which scans the sky for changes using Earth-based telescopes. The explosion shocked astronomers by becoming up to 100 times brighter than a normal supernova, yet it faded in just a few days. Typical supernovae take weeks or even months to dim. What made the Cow even more unusual was its flat shape, as observed by researchers at the University of Sheffield in the UK.
Anna Ho, an astronomer at Cornell University, explains that these events are labeled LFBots because of their brightness (luminous), blue color (from temperatures around 40,000C or 72,000F), and brief visibility in the optical light spectrum. Since 2018, similar explosions have been found and named using animal-themed nicknames based on their original designations. For example, ZTF18abvkwla from 2018 is known as the Koala, ZTF20acigmel from 2020 is the Camel, AT2022tsd from 2022 is the Tasmanian devil, and 2023's AT2023fhn is called the Finch or Fawn.
In November, a new LFBot was discovered by Ho and Daniel Perley of Liverpool John Moores University. Labeled AT2024wpp, it has not been officially nicknamed yet, but the researchers considered calling it "the Wasp." It is the brightest LFBot since the Cow and was spotted very early in its brightening phase, allowing astronomers to focus multiple telescopes, including the Hubble Space Telescope, on the explosion. Perley called it "the best one since the Cow itself."
Originally, scientists thought LFBots might be failed supernovae, where a star collapses inward instead of exploding outward, forming a black hole or neutron star that sends jets of radiation through the star's outer shell. However, the Wasp lacked the outward-flowing material typical of such a scenario. While the data is still being analyzed, this casts doubt on the failed supernova theory.
Another explanation gaining ground is that these explosions happen when intermediate mass black holes—those between 100 and 100,000 times the mass of our Sun—swallow nearby stars. This idea was supported by a study in September 2024 by Zheng Cao at the Netherlands Institute for Space Research. Cao and his team re-examined the Cow using X-ray data and found a disk of material around the explosion, suggesting a star was being eaten by a mid-sized black hole. The bright flares we see from Earth may come from chunks of the star being devoured.
Some scientists, like Brian Metzger from Columbia University, suggest that LFBots might instead come from a different type of interaction. He believes a massive Wolf-Rayet star could be torn apart by a smaller black hole—just 10 to 100 times the mass of our Sun. This would be similar to black hole pairs, which have been observed through gravitational waves.
Despite competing theories, the idea of intermediate mass black holes is currently the most exciting. These mysterious objects could be the missing link between small black holes formed from dead stars and supermassive ones found at the centers of galaxies, like the one in our Milky Way. Confirming their existence would be a major discovery in astrophysics and could even help explain the nature of dark matter.
"The intermediate mass black hole model is the most exciting," says Perley. However, he adds that evidence so far is limited. To better understand LFBots, astronomers need to find and study more of them. Perley estimates that a sample size of 100 would be ideal. A major boost in data might come in 2025 when Israel launches the Ultraviolet Transient Astronomy Satellite (Ultrasat). With a large field of view—covering 204 square degrees, about the size of 1,000 full Moons—Ultrasat could detect many more LFBots.
Other telescopes, like the James Webb Space Telescope (JWST), could also provide valuable data, but securing time to observe LFBots has been difficult. Ho mentioned that she has already proposed JWST observations twice but hasn't been approved yet. She plans to try again.
Until more observations become available, these cosmic explosions remain a fascinating mystery. As Perley puts it, "I thought this was going to be a fun, one-off project. But it turned out to really be this completely separate type of phenomenon. They have become more and more interesting."
The story of LFBots—and the black holes that may cause them—is just beginning.
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