How well do we understand the Universe if we struggle to understand its most energetic events? This question can trigger a wide-ranging philosophical or even epistemological discussion. It's the kind of question that can bring the Universe's most mysterious incidents into the foreground of busy lives.
While the Big Bang is, without a doubt, the most energy-dense event in the Universe's history, there are other energetic events that lack clear causes. Scientists have a pretty clear theoretical framework for the Big Bang, but when it comes to Gamma-Ray Bursts (GRB), our understanding of them is lacking. Unlike the Big Bang, which happened more than 13 billion years ago, GRBs are happening right now, yet scientists don't have a clear understanding of their underlying physics.
Usually in science, discovering more examples of a class of objects leads to greater understanding. However, that may not be the case when it comes to the most recently detected GRB. It's unlike any previous GRB that scientists have encountered.
The new GRB is called GRB 250702B and it was discovered on July 2nd, when the Fermi Gamma-Ray Space Telescope detected it and sent out an alert. But this detection was different. Rather than a single burst, there were three separate bursts over a nearly day-long period.
The detection is presented in new research in The Astrophysical Journal Letters titled "The Day-long, Repeating GRB 250702B: A Unique Extragalactic Transient." Andrew Levan, an astronomer at Radboud University in the Netherlands, is a co-lead author along with Antonio Martin-Carrillo from the School of Physics and Centre for Space Research at the University College Dublin.
GRBs are extremely energetic explosions, the most energetic in the Universe, and they usually last only milliseconds, or sometimes several minutes. They're likely caused by massive stellar explosions or by supermassive black holes tearing huge stars apart. But this GRB's multiple bursts, that span almost an entire day, set it apart from other GRBs. In a press release, Martin-Carrillo said that the GRB is "unlike any other seen in 50-years of GRB observations." Lead author Levan also pointed out how unusual this GRB is. "This is 100-1000 times longer than most GRBs,” he said.
The long-duration, multi-burst nature of the GRB is strange since GRBs come from explosions or disruptions that destroy an object. "More importantly, gamma-ray bursts never repeat since the event that produces them is catastrophic,” said Martin-Carrillo.
Another surprise came when astronomers realized that GRB 250702B was also active in x-rays the day prior to Fermi detecting it. A GRB of this duration, with its multiple bursts, has never been seen before.
This figure from the research shows how the Fermi Gamma-Ray Burst Monitor (GBM) detected GRB 250702B. It shows the three separate bursts labelled GRBs 250702D, 250702B, and 250702E, with each one isolated in a separate top panel. It also shows how the Einstein Probe Wide-field X-ray Telescope (EP-WXT) detected x-ray emissions the day before the GRB. Image Credit: Levan and Martin-Carrillo et al. 2025. ApJL
Initially, astronomers thought the GRB was coming from somewhere in the Milky Way. But observations with the Very Large Telescope’s HAWK-I camera suggested it was extragalactic, and follow-up observations with the Hubble confirmed that. The source is likely a few billion light-years away, meaning that the burst is even more energetic than thought. "Before these observations, the general feeling in the community was that this GRB must have originated from within our galaxy. The VLT fundamentally changed that paradigm,” said Levan.
The question is, what's behind this strange GRB?
"We discuss several possible scenarios to explain the exceptional properties of the burst, which suggest that either a very unusual collapsar or the tidal disruption of a white dwarf by an intermediate-mass black hole are plausible explanations for this unprecedented GRB," the authors write in their research article.
Astronomers aren't completely certain what causes GRBs and what the progenitors are. One problem is that they're so far away that studying them in detail is challenging. "They are attributed to the catastrophic outcomes of stellar-scale events and, as such, are not expected to recur," the authors write.
Astronomers think that GRBs can be created when massive stars explode as supernovae or superluminous supernovae and collapse into black holes. These are called collapsars. "If this is a massive star, it is a collapse unlike anything we have ever witnessed before,” said Levan. If that were the case, then the GRB wouldn't have lasted so long. Another explanation is when a massive star is being torn apart by a black hole, which could produce a GRB lasting an entire day. But that explanation can't account for other properties of GRB 250702B.
GRBs are typically followed by an afterglow in longer wavelengths as the ejecta from the GRB collides with interstellar gas. "The multiwavelength counterpart is well described with standard afterglow models at a relatively low redshift z ∼ 0.3, but the prompt emission does not readily fit within the expectations for either collapsar or merger-driven GRBs," the authors write in their research. "Indeed, a striking feature of the multiple prompt outbursts is that the third occurs at an integer multiple of the interval between the first two." This could indicate periodicity in the progenitor system, but that's not conclusive.
Because the GRB is confirmed to be extragalactic, that rules out X-ray binaries, cataclysmic variables, novae, and magnetars as progenitors. They simply wouldn't be this energetic at such a great distance.
One candidate is a relativistic tidal disruption event (TDE), which are known to emit high-energy GRBs for hours or even days. Since the GRB is not located in the nucleus of the host galaxy, that eliminates a TDE involving a supermassive black hole.
These images from the research show the location of GRB 250702B. The second two panels show the GRB offset from the nucleus of its host galaxy, which is just to the right. "The transient appears to be offset from a likely extended source," the authors explain. The right-most image is from the Hubble, and it reveals "a complex, asymmetric morphology, as well as the possible presence of a strong dust lane through the disk of the galaxy," according to the authors. Image Credit: Levan and Martin-Carrillo et al. 2025. ApJL
The GRB could be from a TDE involving a white dwarf (WD) and an intermediate mass black hole (IMBH), because that could explain the periodicity, according to the researchers. "In the TDE scenario, we thus favor the WD-IMBH explanation," the authors write.
Another possibility is a collapsar. "The majority of long-duration GRBs appear to arise from the core collapse of massive stars," the authors write. But they also point out that some can come from mergers of compact objects like WDs and BHs.
Not everything about GRB 250702B agrees with a collapsar progenitor, while some aspects do. "In the case of GRB 250702B, the combination of morphology (starting faint and getting brighter) and prompt energy correlations further challenge the interpretation of GRB 250702B as a core-collapse event," the researchers explain. They also write that "Although several lines of evidence point away from a collapsar, it should also be noted that several others are entirely in keeping with a collapsar origin."
Almost everything about GRBs is uncertain. Attempts to categorize them often result in blurred lines between the different potential progenitors. That's why this discovery is so puzzling and valuable at the same time.
"GRB 250702B is observationally unprecedented in its timescale, morphology, and the onset of X-ray photons prior to the initial GRB trigger," the authors write in their conclusion.
After exploring several possibilities, they write "We find that an atypical collapsar and a possibly repeating WD-IMBH TDE are most compatible with current available information." But they also leave the door open to other possibilities.
"Alternatively, given its unprecedented nature, does it arise from a potentially previously unseen kind of object?" the authors ask in their paper. "As a rare event it is also possible that unusual and rare circumstances are at play in GRB 250702B that do not impact the general GRB population, including GRBs originating at unusual phases of stellar evolution."
GRBs can emit as much energy in a single burst as the Sun will in its entire, multi-billion year history. The fact that we still don't have a thorough understanding of them is a little humbling. However, finding this rare outlier is a sign that we're making progress. Eventually, outliers like it will either break or strengthen our theoretical explanations for GRBs.
The researchers are still watching GRB 250702B with other telescopes, including the JWST and the VLT’s X-shooter spectrograph. Those observations will help constrain the nature of the progenitor, and maybe even come up with a definitive answer. But for now, there's no concrete explanation for this extremely unusual GRB.
“We are still not sure what produced this, but with this research we have made a huge step forward towards understanding this extremely unusual and exciting object,” said Martin-Carrillo.
