Earlier this year, astronomers watched a burst of high-energy light that kept roaring back for nearly a full day. Gamma-ray bursts (GRBs) are usually sorted into short and long events, and they generally last from milliseconds to a few minutes.

In a new study, researchers describe a signal named GRB 250702B that fired three distinct times over a few hours, with soft X-rays flaring even earlier. The team also reported evidence that the source lies beyond our galaxy.

Lead researchers Antonio Martin-Carrillo of University College Dublin (UCD) and Andrew J. Levan of Radboud University (RU) directed the effort.

Their teams combined rapid-response alerts from NASA’s Fermi satellite, China’s Einstein Probe, and follow-up observations with the Very Large Telescope (VLT) and the Hubble Space Telescope.

Martin-Carrillo called it “unlike any other seen in 50 years of GRB observations. This is 100 to 1,000 times longer than most GRBs. More importantly, gamma-ray bursts never repeat since the event that produces them is catastrophic.” 

What makes GRB 250702B so strange

GRBs don’t typically don’t happen twice, much less three or more times. Here, Fermi saw three distinct outbursts separated by thousands of seconds, while the Einstein Probe caught earlier X-ray activity that started many hours before.

The timing was not random. The third outburst arrived at roughly an integer multiple of the gap between the first two. This pattern suggests at a repeating process inside the source rather than a single one-off blast.

The overall energy in gamma rays is in the same ballpark as other GRBs at similar distances, yet the pattern of activity is different.

Standard relations that link a burst’s peak energy to its total output do not align cleanly for each of the three pulses.

The fading afterglow across X-ray, infrared, and radio bands looks normal for a jet from a compact central engine. The near-infrared signal was unusual in one respect: it was extremely red, which suggests heavy dust near the site.

Locating a cosmic mystery

“Before these observations, the general feeling in the community was that this gamma-ray burst must have originated from within our galaxy,” said Levan. “The VLT fundamentally changed that paradigm.”

VLT’s HAWK-I camera picked up an extremely red point of light that faded over days.

Hubble went a step further and resolved a galaxy at the same spot. That finding places the source in an extragalactic system, likely several billion light-years away. The engine must be extremely powerful to be visible at all.

VLT’s X-shooter spectrograph covered visible to infrared wavelengths and saw a very faint, red continuum, consistent with heavy dust.

While the exact redshift is still being refined, the data point to a relatively nearby cosmic distance compared with many GRBs, which helps explain the sharp view of the host galaxy.

Potential black hole link

One explanation for GRB 250702B points to a jetted tidal disruption event, in which a black hole tears apart a passing star and feeds on the debris.

Astronomers have observed such jets firing erratically in events like Swift J1644+57, described in a paper that modeled how a powerful magnetic field around the black hole could drive on-off behavior.

Another possibility fits the unusual timing. The authors consider a compact white dwarf star passing close to an intermediate-mass black hole.

This class of black holes is heavier than stellar-mass ones but lighter than the giants in galaxy centers, with masses around 100 to 100,000 times that of the Sun.

In this scenario, the star could be partially stripped during each close pass, feeding the jet in bursts spaced according to the star’s orbit.

Doesn’t fit the mold

A more familiar GRB engine, a collapsing massive star, is not off the table.

The problem is explaining how such a collapse would produce multiple widely spaced pulses over many hours rather than one sustained episode lasting seconds to minutes.

Lensing by a foreground galaxy can also repeat a transient by splitting light paths, but the pulse shapes here differ from one another. The color and afterglow behavior also point away from that lensing explanation.

Light patterns from GRB 250702B

The first detections were in soft X-rays, then came the brightest gamma-ray flash hours later. That order flips the usual script for GRBs, where the strongest gamma rays typically arrive first and fade quickly.

Infrared light faded quickly, and radio telescopes detected a faint source consistent with a standard jet interacting with surrounding gas.

Those observations match the afterglow physics well, even if the prompt high-energy behavior does not fit neatly into existing categories.

The host galaxy is not a nuclear site, which weakens ideas that require a supermassive black hole sitting at the center. If the burst is a disruption event, an off-center intermediate-mass black hole is a better match for the position.

The red color likely comes from dust lanes in the host galaxy that absorb blue light. That dust also lines up with the strong X-ray absorption measured from the same line of sight.

Watching for a supernova

In jet-driven tidal disruption events, X-ray light can plummet once the jet shuts off, while radio emission can rise for months as the blast wave expands.

Continued X-ray and radio monitoring will test whether this source follows that playbook.

If a supernova is involved, it would glow in the infrared a few weeks after the collapse. Dust makes that search harder, but large, sensitive telescopes can still look for the telltale rise and fall.

Spectroscopy can also help determine the distance and characterize the environment around the source.

A firm redshift would establish the energy budget and sharpen the comparison to known gamma-ray bursts and disruption events.

“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.

The study is published in The Astrophysical Journal Letters.

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