Astronomers have identified the likely cause behind mysterious cosmic flashes known as luminous fast blue optical transients (LFBOTs). These rare events, first observed in 2014, are characterized by brief but intense bursts of blue and ultraviolet light, followed by faint X-ray and radio emissions. The nature of LFBOTs has been debated for years, with some suggesting they result from unusual supernovae or from interstellar gas falling into black holes.
A team led by researchers at the University of California, Berkeley has now concluded that LFBOTs originate from extreme tidal disruption events. In these cases, a black hole up to 100 times the mass of the sun destroys its massive star companion within days.
Raffaella Margutti, associate professor of astronomy and physics at UC Berkeley, explained: “Theorists have come up with many ways to explain how we get these large black holes, to explain what LIGO sees. 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 latest event studied, AT 2024wpp, was analyzed in two papers recently accepted by The Astrophysical Journal Letters. The research involved detailed examination of X-ray, radio, optical, ultraviolet and near-infrared emissions. Nayana A.J., a postdoctoral fellow at UC Berkeley, led the analysis of X-ray and radio data while graduate student Natalie LeBaron focused on other wavelengths.
The energy released during AT 2024wpp was found to be about 100 times greater than that produced by typical supernovae. This ruled out stellar explosions as an explanation for these flashes. As LeBaron noted: “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. 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 accumulated material from its companion star into a surrounding halo. When the companion finally approached too closely and was torn apart, debris formed an accretion disk around the black hole and interacted with existing material there. This process generated high-energy radiation including X-rays and ultraviolet light. Some gas was ejected as jets moving at about 40% of light speed.
The shredded companion star is estimated to have been more than ten times as massive as our sun—possibly a Wolf-Rayet star—and located in a galaxy actively forming new stars about 1.1 billion light-years away.
Multiple telescopes contributed observations across various wavelengths for this study: NASA’s Chandra X-ray Observatory; Swift-XRT; NuSTAR; ALMA; Australia Telescope Compact Array; UVOT on NASA’s Neil Gehrels Swift Observatory; Keck Observatory; Lick Observatory; and Gemini Observatories.
Future discoveries may become more common thanks to planned ultraviolet space telescopes such as ULTRASAT and UVEX—projects involving scientists from UC Berkeley—which will help astronomers detect LFBOTs earlier in their evolution.
Nayana A.J. commented: “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.”
Margutti’s research received support from the National Science Foundation and NASA.
For further information:
– The Most Luminous Known Fast Blue Optical Transient AT2024wpp: Unprecedented Evolution and Properties in the Ultraviolet to the Near-Infrared (arXiv)
– The Most Luminous Known Fast Blue Optical Transient AT2024wpp: Unprecedented Evolution and Properties in the X-rays and Radio (arXiv)
– Chandra X-ray Observatory announcement
– Raffaella Margutti lab website
