Black Hole Flare is Biggest and Most Distant Seen

The most massive stars in the universe are destined to explode as brilliant supernova before collapsing into black holes. Yet one huge star appears to have never fulfilled its destiny; in a twist of irony, the star wandered too close to a gargantuan black hole, which gobbled it up, shredding the star to bits and pieces.

That is the most likely explanation to come from authors of a new Nature Astronomy report describing the most powerful and most distant flare of energy ever recorded from a supermassive black hole. The cosmic object was first observed in 2018 by the US National Science Foundation (NSF)-funded Zwicky Transient Facility (ZTF), based at Caltech's Palomar Observatory, and the Caltech-led Catalina Real-Time Transient Survey, which is also funded by NSF. The flare rapidly brightened by a factor of 40 over a period of months, and, at its peak, was 30 times more luminous than any previous black hole flare seen to date. At its brightest, the flare shined with the light of 10 trillion suns.

The supermassive black hole behind the flare is a type of accreting, or feeding, black hole called an active galactic nucleus (AGN). Referred to as J2245+3743, this AGN is estimated to be 500 million times more massive than our Sun. It resides 10 billion light-years away in the remote universe. Because light has a finite speed and takes time to reach us, astronomers observe distant events like this one in the past, when the universe was young.

"The energetics show this object is very far away and very bright," says study lead author Matthew Graham, research professor of astronomy at Caltech, as well as the project scientist for ZTF, and a co-principal investigator of the project. "This is unlike any AGN we've ever seen."

Matthew Graham Credit: Nick Macdonald

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Matthew Graham

Credit: Nick Macdonald

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Astronomers are continuing to monitor the black hole flare though it is fading over time. In fact, in addition to witnessing the object in the past, time itself runs slower at the remote site of the black hole compared to our own experience of time. "It's a phenomenon called cosmological time dilation due to stretching of space and time. As the light travels across expanding space to reach us, its wavelength stretches as does time itself," Graham explains, noting that long-lived surveys like ZTF and Catalina are important to fully witness events in the past because, in this case, "seven years here is two years there. We are watching the event play back at quarter speed."

To determine what could cause such a dramatic burst of light in the cosmos, the researchers thoroughly examined a list of possibilities, concluding that the most likely culprit is a tidal disruption event (TDE). This phenomenon occurs when a supermassive black hole's gravity shears a star that comes too close, slowly consuming the star over time as it spirals into the black hole. The fact that the black hole flare J2245+3743 is still going indicates that we are witnessing a star not yet fully devoured but rather like "a fish only halfway down the whale's gullet," Graham says.

If the flare is from a TDE, the scientists estimate that the supermassive black hole gobbled a star with a mass at least 30 times greater than that of our Sun. The previous record holder for the largest candidate TDE, an event nicknamed Scary Barbie after its initial ZTF classification as ZTF20abrbeie, was not nearly as intense. That TDE, which is also thought to have originated from an AGN, was 30 times weaker than that of J2245+3743, and its doomed star is estimated to have been between three and 10 solar masses.

Stellar Snack Within a Black Hole's Disk

Most of the roughly 100 TDEs seen to date do not take place around AGN—massive structures that consist of supermassive black holes surrounded by large, swirling disks of material that feed the central black hole. The AGN burble along, flaring up with their own feeding activity, which can mask TDE bursts and makes them harder to find. The recent jumbo flare J2245+3743, on the other hand, was so large that it was easier to see.

However, at first, J2245+3743 did not seem to be anything special. In 2018, after the object was first spotted, the researchers used the 200-inch Hale Telescope at Caltech's Palomar Observatory to obtain a spectrum of the object's light, but it did not reveal anything unusual. In 2023, the team noticed the flare was decaying slower than expected, so they obtained another spectrum from the W. M. Keck Observatory in Hawai‘i, which indicated the extreme brightness of this particular AGN.

"At first, it was important to establish that this extreme object was truly this bright," explains co-author K. E. Saavik Ford, a professor at the City University of New York (CUNY) Graduate Center and Borough of Manhattan Community College and American Museum of Natural History (AMNH). It was possible, she says, that the object could have been beaming the light toward us rather than glowing in all directions, but data from NASA's former Wide-field Infrared Survey Explorer (WISE) mission helped rule that out. In the end, after other scenarios were also ruled out, the researchers concluded that J2245+3743 was indeed the brightest black hole flare ever recorded.

"If you convert our entire Sun to energy, using Albert Einstein's famous formula E = mc2, that's how much energy has been pouring out from this flare since we began observing it," Ford says.

Once the team established the unprecedented brightness of the event, they looked at what could possibly have caused it. "Supernovae are not bright enough to account for this," Ford says, referring to one possibility. Instead, the team's favored explanation is a supermassive black hole slowly ripping a huge star to death.

"Stars this massive are rare," Ford says, "but we think stars within the disk of an AGN can grow larger. The matter from the disk is dumped onto stars, causing them to grow in mass."

Finding a black hole meal with such mega proportions indicates that other events like this are likely taking place across the cosmos. The researchers hope to mine through more ZTF data to find others, and the NSF and Department of Energy's Vera C. Rubin Observatory may likewise find unusually large TDEs.

"We never would have found this rare event in the first place if it weren't for ZTF," Graham says. "We've been observing the sky with ZTF for seven years now, so when we see anything flare or change, we can see what it has done in the past and how it will evolve."

The Nature Astronomy study titled "An Extremely Luminous Flare Recorded from a Supermassive Black Hole" was funded by the NSF, the Simons Foundation, NASA, and the German Research Foundation. Other Caltech authors include Andrew Drake, Yuanze Ding (MS '25), Mansi Kasliwal (PhD '11), Sam Rose, Jean Somalwar (now a postdoc at UC Berkeley), George Djorgovski, Shri Kulkarni, and Ashish Mahabal; Tracy Chen and Steven Groom of Caltech's IPAC astronomy center; and Daniel Stern of NASA's Jet Propulsion Laboratory (which is managed by Caltech). Additional authors are Barry McKernan of CUNY Graduate Center and Borough of Manhattan Community Collegeand AMNH; Matteo Cantiello of the Simons Foundation's Flatiron Institute and Princeton University; Mike Koss of Eureka Scientific; Raffaella Margutti of UC Berkeley; Phil Wiseman of University of Southampton, UK; Patrik Veres of Ruhr University in Bochum, Germany; and Eric Bellm of the University of Washington.

Caltech's ZTF is funded by the NSF and an international collaboration of partners. Additional support comes from the Heising-Simons Foundation and from Caltech. ZTF data are processed and archived by Caltech's IPAC. NASA supports ZTF's search for near-Earth objects through the Near-Earth Object Observations program.