Data acquired with multiple NSF NOIRLab facilities indicate gamma-ray burst lasting over seven hours resides in a massive, extremely dusty galaxy
Astronomers have observed the longest-ever gamma-ray burst — a powerful, extragalactic explosion that lasted over seven hours. Rapid follow-up observations with the U.S. Department of Energy-fabricated Dark Energy Camera and the International Gemini Observatory, funded in part by the U.S. National Science Foundation and operated by NSF NOIRLab, provided crucial information about the possible origin of this extraordinary event and the galaxy that hosts it.
Gamma-ray bursts (
GRBs) are among the most powerful explosions in the Universe, second only to the
Big Bang. The majority of these bursts are observed to flash and fade within a few seconds to minutes. But on 2 July 2025, astronomers were alerted to a GRB source that was exhibiting repeating bursts and would end up lasting over seven hours. This event, dubbed
GRB 250702B, is the longest gamma-ray burst humans have ever witnessed.
GRB 250702B was first identified by NASA’s Fermi Gamma-ray Space Telescope (
Fermi). Shortly after space-based telescopes detected the initial bursts in gamma-rays and pinpointed its on-sky location in X-rays, astronomers around the world launched campaigns to observe the event in additional
wavelengths of light.
One of the first revelations about this event came when infrared observations acquired by ESO's Very Large Telescope (
VLT)
established that the source of GRB 250702B is located in a galaxy outside of ours, which until then had remained a question.
Following this, a team of astronomers led by Jonathan Carney, graduate student at the University of North Carolina at Chapel Hill, set out to capture the event’s evolving afterglow, or the fading light emissions that follow the initial, extremely bright flash of gamma-rays. The properties of these emissions can provide clues about the type of event that caused the GRB.
To better understand the nature of this record-breaking event, the team used three of the world’s most powerful ground-based telescopes: the
NSF Víctor M. Blanco 4-meter Telescope and the twin 8.1-meter
International Gemini Observatory telescopes
[1]. This trio observed GRB 250702B starting roughly 15 hours after the first detection until about 18 days later. The team presents their findings in a
paper published on 26 November in
The Astrophysical Journal Letters.
The Blanco telescope is located in Chile at NSF Cerro Tololo Inter-American Observatory (
CTIO), a Program of NSF NOIRLab. The International Gemini Observatory consists of the Gemini North telescope in Hawai‘i and the Gemini South telescope in Chile. It is partly funded by NSF and operated by NSF NOIRLab.
“The ability to rapidly point the Blanco and Gemini telescopes on short notice is crucial to capturing transient events such as gamma-ray bursts,” says Carney.
“Without this ability, we would be limited in our understanding of distant events in the dynamic night sky.”
The team used a suite of instruments for their investigation: the
NEWFIRM wide-field infrared imager and the 570-megapixel DOE-fabricated Dark Energy Camera (
DECam), both mounted on the Blanco telescope, and the Gemini Multi-Object Spectrographs (
GMOS) mounted on Gemini North and Gemini South.
Analysis of the observations revealed that GRB 250702B could not be seen in visible light, partly due to interstellar dust in our own Milky Way Galaxy, but more so due to dust in the GRB’s host galaxy. In fact, Gemini North, which provided the only close-to-visible-wavelength detection of the host galaxy, required nearly two hours of observations to capture the faint signal from beneath the swaths of dust.
Carney and his team then combined these data with new observations taken with the Keck I Telescope at
W. M. Keck Observatory, the Magellan Baade Telescope, and the Fraunhofer Telescope at Wendelstein Observatory, as well as publicly available data from VLT, NASA’s Hubble Space Telescope (
HST), and X-ray and radio observatories. They then compared this robust dataset with theoretical models, which are frameworks that explain the behavior of astronomical phenomena. Models can be used to make
predictions that can then be tested against observational data to refine scientists' understanding.
The team’s analysis established that the initial gamma-ray signal likely came from a narrow, high-speed jet of material crashing into the surrounding material, known as a
relativistic jet. The analysis also helped characterize the environment around the GRB and the host galaxy overall. They found that there is a large amount of dust surrounding the location of the burst, and that the host galaxy is extremely massive compared to most GRB hosts. The data support a picture in which the GRB source resides in a dense, dusty environment, possibly a thick lane of dust present in the host galaxy along the line-of-sight between Earth and the GRB source. These details about the environment
of GRB 250702B provide important constraints on the system that produced the initial outburst of gamma-rays.
Of the roughly 15,000 GRBs observed
since the phenomenon was first recognized in 1973, only a half dozen come close to the length of GRB 250702B. Their proposed origins range from the collapse of a
blue supergiant star, a
tidal disruption event, or a newborn
magnetar. GRB 250702B, however, doesn’t fit neatly into any known category.
From the data obtained so far, scientists have a few ideas of possible origin scenarios: (1) a black hole falling into a star that’s
been stripped of its hydrogen and is now almost purely helium, (2) a star (or sub-stellar object such as a planet or brown dwarf) being disrupted during a close encounter with a stellar compact object, such as a stellar black hole or a neutron star, in what is known as a micro-tidal disruption event, (3) a star being torn apart as it falls into an
intermediate-mass black hole — a type of black hole with a mass ranging from one hundred to one
hundred thousand times the mass of our Sun that is believed to exist in abundance, but has so far been
very difficult to find. If it is the latter scenario, this would be the first time in history
that humans have witnessed a relativistic jet from an intermediate mass black hole in the act of consuming a star.
While more observations are needed to conclusively determine the cause of GRB 250702B, the data acquired so far remain consistent with these novel explanations.
“This work presents a fascinating cosmic archaeology problem in which we’re reconstructing the details of an event that occurred
billions of light-years away,” says Carney.
“The uncovering of these cosmic mysteries demonstrates how much we are still learning about the Universe's most extreme events and reminds us to keep imagining what might be happening out there.”
