The Bizarre Space Explosions Scientists Can't Explain

Astronomers have spotted around a dozen of these weird, rare blasts. Could they be signs of a special kind of black hole?

Astronomers had never seen anything like it before – something vast, in the depths of space, went boom. Telescopes on Earth picked up the astonishingly bright and unusual explosion in 2018, watching as it played out 200 million light years away.

The blast brightened rapidly and brilliantly, far more than one would expect for a regular star explosion, a supernova, before it disappeared. Given the label AT2018cow, the weird-looking blast – which was about the same size as our solar system – soon become known by a more memorable nickname: "the Cow".

Since this dazzling event, astronomers have detected a handful of other, similar explosions across the cosmos. Described as luminous fast blue optical transients (LFBots), they all share the same characteristics.

"They are very luminous," says Anna Ho, an astronomer at Cornell University in New York, hence the L in LFBot. The blue colour is a result of the explosion's extraordinarily high temperature of around 40,000C (72,000F), which shifts light to the blue part of the spectrum. The last letters in the acronym, O and T, refer to the fact these events appear in the visible light spectrum (optical) and are short-lived (transient).

Initially, scientists thought LFBots might be failed supernovae, stars that tried to explode but instead imploded, forming a black hole at their core that subsequently consumed them from the inside-out.

However, another theory is gaining traction – that cow flares are triggered when an undiscovered class of mid-sized black holes, known as intermediate mass black holes, swallow up stars that stray too close to them. A research paper published in November last year described fresh evidence for this idea, suggesting that it might be the preferred explanation. "The general sentiment is shifting in that direction," says Daniel Perley, an astronomer at Liverpool John Moores University.

If this turns out to be correct, it could provide vital proof of the existence of this elusive type of black hole – a missing link between the smallest and largest black holes in the Universe and a vital clue to one of its biggest mysteries, dark matter.

The original cow flare in 2018 was detected by a robotic survey of the sky that uses Earth-based telescopes called the Asteroid Terrestrial-impact Last Alert System (Atlas). This system caught the explosion occurring in a galaxy about 200 million light-years from Earth. Noticeably, it was up to 100 times brighter than a regular supernova, and it came and went in just a few days. Normal supernovae take weeks or even months to run their course. This explosion also had a strange, flat structure, according to a observations by researchers at the University of Sheffield in the UK.

Since then, astronomers have identified about a dozen similar events. Most have also become known by animal-themed nicknames loosely based on the letters they are randomly assigned by the astronomical survey that discovers them. ZTF18abvkwla, also witnessed in 2018, is called the Koala. ZTF20acigmel, spotted in 2020, is known as the Camel. AT2022tsd, found in 2022, is the Tasmanian devil. And 2023's AT2023fhn is called the Finch or Fawn.

Astronomers have increasingly been using telescope surveys to study large portions of the sky in an effort seek out these events. When one flares up, they can then send an alert out to other astronomers on services such as the Astronomer's Telegram, a kind of online message board for astronomical events. Such alerts aim to prompt other telescopes into action, pointing them towards the event in the hope of observing it in detail before the blast fades from view.

The Hubble Space Telescope detected the brief flash of a LFBot in 2023
 that has been nicknamed "the Finch" due to its designation AT2023fhn
(Credit: Nasa)

- Author: Jonathan O'Callaghan, BBC