Astronomers have identified the cause of rare, bright blue cosmic flashes known as luminous fast blue optical transients (LFBOTs), according to research led by the University of California, Berkeley. These flashes, first observed in 2014 and only seen slightly more than a dozen times since, had puzzled scientists who debated whether they were due to supernovae or gas falling into black holes.
A recent analysis of AT 2024wpp, the brightest LFBOT detected so far, suggests that these phenomena result from an extreme tidal disruption event. In this scenario, a black hole up to 100 times the mass of the sun destroys its massive stellar companion within days.
“Theorists have come up with many ways to explain how we get these large black holes, to explain what LIGO sees,” said Raffaella Margutti, associate professor of astronomy and physics at UC Berkeley. “LFBOTs allow you to get at this question from a completely different angle. They also allow us to characterize the precise location where these things are inside their host galaxy, which adds more context in trying to understand how we end up with this setup — a very large black hole and a companion.”
The energy emitted by AT 2024wpp was calculated to be about 100 times greater than what would occur in a normal supernova. This amount of energy requires converting roughly 10% of the sun’s rest-mass into energy over weeks—a scale not possible through standard stellar explosions.
“The sheer amount of radiated energy from these bursts is so large that you can’t power them with the collapse and explosion of a massive star — or any other type of normal stellar explosion,” said Natalie LeBaron, graduate student at UC Berkeley. “The main message from AT 2024wpp is that the model that we started off with is wrong. It’s definitely not caused by an exploding star.”
Researchers propose that over time, the black hole had siphoned material from its companion star, building up a surrounding halo. When the companion finally drew too close and was shredded apart, debris formed an accretion disk around the black hole. The interaction between new debris and existing material generated X-ray, ultraviolet (UV), and blue light emissions. Gas from the companion star also formed jets moving at about 40% the speed of light; when these jets hit nearby gas, they produced radio waves.
Analysis indicates that the destroyed star likely exceeded ten solar masses and may have been a Wolf-Rayet star—hot stars with weak hydrogen signatures—which matches observations made for AT 2024wpp.
AT 2024wpp lies in a galaxy experiencing active star formation approximately 1.1 billion light-years away and is significantly more luminous than previous LFBOTs such as AT 2018cow.
To study various wavelengths emitted during such events, astronomers used multiple telescopes including NASA’s Chandra X-ray Observatory, Swift-XRT, NuSTAR; radio arrays like ALMA and CSIRO’s Australia Telescope Compact Array; UV/optical telescopes on NASA’s Neil Gehrels Swift Observatory; and ground-based observatories such as Keck, Lick, and Gemini.
Looking ahead, researchers expect future UV space telescopes—ULTRASAT and UVEX—to enable routine detection of LFBOTs by capturing them before peak brightness. These missions involve several UC Berkeley scientists through operations at the Space Sciences Laboratory.
“Right now, we find only about one LFBOT per year. But once we have UV telescopes in place in space, then finding LFBOTs will become routine, like detecting gamma ray bursts today,” said Nayana A.J., postdoctoral fellow at UC Berkeley.
Margutti’s work received support from both the National Science Foundation and NASA.
