Astronomers have observed approximately a dozen unusual and infrequent cosmic explosions, leading to speculation about a possible connection to a unique type of black hole. These events, initially unprecedented, involve massive and powerful eruptions in the distant reaches of space. Specifically, in 2018, telescopes on Earth detected an exceptionally bright and peculiar explosion occurring 200 million light-years away.
This explosion, far exceeding the typical brightness of a supernova, rapidly intensified before fading away, earning the designation AT2018cow and the nickname “the Cow” due to its unusual appearance and size, comparable to our solar system. Subsequent to this remarkable phenomenon, astronomers have identified several similar explosions throughout the universe, classified as luminous fast blue optical transients (LFBOTs), which exhibit common characteristics. According to Anna Ho, an astronomer at Cornell University, the “L” in LFBOT denotes their extreme luminosity.
Furthermore, the blue hue arises from the extraordinarily high temperatures, around 40,000°C (72,000°F), which cause a shift in light toward the blue end of the spectrum. The “O” and “T” signify that these explosions are observed in the visible light spectrum (optical) and are transient, meaning they are short-lived.
Originally, scientists hypothesized that LFBOTs were caused by failed supernovae, where stars attempted to explode but instead collapsed, creating a central black hole that devoured them from within. However, a competing hypothesis suggests that cow flares are caused by intermediate-mass black holes, a previously undetected class, consuming stars that venture too near.
Recent research, published last November, strengthens this theory, suggesting it may be the more accurate explanation, a view increasingly shared by astronomers like Daniel Perley. If proven true, this could confirm the existence of these elusive intermediate-mass black holes, bridging the gap between the smallest and largest black holes and providing crucial insight into the enigma of dark matter.
In 2018, the Asteroid Terrestrial-impact Last Alert System (Atlas), a robotic sky survey using ground-based telescopes, discovered an unusual stellar flare. Atlas spotted the outburst in a galaxy roughly 200 million light-years away. The event was remarkably bright, peaking at 100 times the luminosity of a typical supernova, and its duration was exceptionally short, lasting only a few days compared to the weeks or months of a standard supernova. Further observations by University of Sheffield researchers revealed that the explosion possessed an unusual, flattened configuration.
Following the initial discovery, astronomers have found approximately a dozen comparable occurrences. These events are often given informal, animal-related nicknames, generally inspired by the random letter combinations assigned to them by the astronomical survey that first identifies them. For instance, ZTF18abvkwla, observed in 2018, is nicknamed the Koala; ZTF20acigmel, detected in 2020, is called the Camel; AT2022tsd, discovered in 2022, is the Tasmanian Devil; and AT2023fhn, from 2023, goes by either the Finch or Fawn.
To find these transient events, astronomers are increasingly relying on large-scale telescope surveys which scan vast areas of the sky. Upon the detection of a flare-up, an alert is disseminated to other astronomers through platforms like the Astronomer’s Telegram, an online forum for astronomical discoveries. These alerts are designed to encourage follow-up observations from other telescopes, directing them to the event in order to capture detailed data before the outburst diminishes.
In November, Ho and Perley identified AT2024wpp, a new luminous fast blue optical transient (LFBOT) awaiting a nickname, with Ho suggesting “the Wasp.” This event garnered significant interest due to its exceptional brightness, surpassing all other LFBOTs since the Cow, and its early detection, enabling extensive observation with telescopes like Hubble.
Perley remarked that it is the most remarkable LFBOT observed since the Cow itself. Preliminary data suggests that, unlike the hypothesized failed supernova scenario involving a collapsing star, the Wasp might have a different origin. The failed supernova theory posits the formation of a black hole or neutron star within the collapsing star, producing jets of radiation that create the brief flare observed as the Cow.
But the Wasp seemed to lack any sign of material flowing away from the explosion that scientists would expect from such an event, says Perley. However, he notes that current findings are preliminary. “We’re still analysing the data,” he says.
It looked like the remnants of a star that was being eaten by an intermediate mass black hole
In September 2024, Zheng Cao and collaborators at the Netherlands Institute for Space Research revisited the initial LFBot discovery, uncovering data that cast doubt on the previously held failed supernova explanation. Their analysis of X-ray observations revealed a potential disk of matter encircling the explosion site. Computer simulations of this disk suggested it was the debris of a star undergoing tidal disruption by an intermediate-mass black hole, possessing a mass estimated at 100 to 100,000 times that of our Sun.
(For perspective, supermassive black holes can reach masses millions or even billions of times greater than the Sun – explore the scale of these cosmic giants in this article about the largest black holes in the Universe.) The consumption of the star by the black hole would periodically trigger sudden bursts of brightness, corresponding to the “Cow” flares observed by astronomers on Earth as larger stellar fragments were swallowed.
“I believe our study supports the intermediate mass black holes nature of AT2018cow and similar LFBots,” says Cao.
One alternative explanation posits that LFBOTs are Wolf-Rayet stars, massive stars in the process of being disrupted by smaller black holes, with masses ranging from 10 to 100 times that of the Sun. This theory finds support from researchers like Brian Metzger, a theoretical astrophysicist at Columbia University. He suggests that these systems could originate through a process analogous to the formation of binary black holes, previously observed via their gravitational waves, with the key difference being that only one of the stars collapses into a black hole.
The intermediate-mass black hole theory currently holds considerable appeal and is a frontrunner in the field, as its confirmation would position LFBots as invaluable tools for investigating these elusive mid-sized black holes. Although astronomers generally believe intermediate-mass black holes exist, conclusive evidence remains elusive. Crucially, these black holes may bridge the gap between the smallest and largest black holes, including supermassive ones like the one residing in the Milky Way’s center. LFBots have the potential to pinpoint the locations of intermediate-mass black holes and determine their prevalence in the universe.
Perley considers the intermediate mass black hole model the most compelling, though its validity remains a point of contention within the scientific community due to limited evidence.
According to Perley, definitive identification of LFBots requires a substantially larger sample size. He laments their scarcity, suggesting that data from approximately 100 LFBots would be the ideal next step. The upcoming launch of the Israeli Ultraviolet Transient Astronomy Satellite (Ultrasat), with its expansive 204-square-degree field of view, holds promise for discovering additional LFBots and other cosmic transients.
Metzger believes the James Webb Space Telescope (JWST) is ideally suited to collect further data on individual LFBots, provided it can be directed towards an explosion during its brightening phase. However, securing observation time on JWST has been challenging. Ho, despite two attempts, has been unsuccessful but intends to submit another proposal this year.
Until more data becomes available, the enigma surrounding these unusual explosions persists. What is certain is that LFBots have revealed themselves to be far more extraordinary than initially anticipated.
Perley reflects on the unexpected depth of the investigation, stating that what began as a lighthearted project has transformed into the study of a distinct and captivating phenomenon.
