Gemini and Blanco telescopes unlock clues to origin of longest gamma-ray burst ever observed

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 the 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 the W. M. Keck 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.”

Notes

[1] This study uses data obtained from several sources, including:

• Gamma-ray Burst Monitor (GBM) on NASA’s Fermi Gamma-Ray Space Telescope (Fermi)
• DOE-fabricated Dark Energy Camera (DECam) and NEWFIRM on the NSF Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO)
• Gemini Multi-Object Spectrographs (GMOS-N and GMOS-S) at the International Gemini Observatory
• Fraunhofer Telescope at Windelstein Observatory
• Hubble Space Telescope (HST)
• Keck I Telescope at the W. M. Keck Observatory
• FourStar on the Magellan Baade Telescope

More information

This research was presented in a paper titled “Optical/infrared observations of the extraordinary GRB 250702B: a highly obscured afterglow in a massive galaxy consistent with multiple possible progenitors” to appear in The Astrophysical Journal Letters. DOI: 10.3847/2041-8213/ae1d67

The team is composed of J. Carney (University of North Carolina at Chapel Hill, USA), I. Andreoni (University of North Carolina at Chapel Hill, USA), B. O'Connor (Carnegie Mellon University, USA), J. Freeburn (University of North Carolina at Chapel Hill, USA), H. Skobe (Carnegie Mellon University, USA), L. Westcott (University of Manchester, UK), M. Busmann (Ludwig Maximilian University of Munich, Germany), A. Palmese (Carnegie Mellon University, USA), X. J. Hall (Carnegie Mellon University, USA), R. Gill (National Autonomous University of Mexico, Mexico/The Open University of Israel, Israel), P. Beniamini (The Open University of Israel, Israel/The George Washington University, USA), E. R. Coughlin (Syracuse University, USA), C. D. Kilpatrick (Northwestern University, USA), A. Anumarlapudi (University of North Carolina at Chapel Hill, USA), N. M. Law (University of North Carolina at Chapel Hill, USA), H. Corbett (University of North Carolina at Chapel Hill, USA), T. Ahumada (California Institute of Technology, USA), P. Chen (Zhejiang University, China), C. Conselice (University of Manchester, UK), G. Damke (NSF NOIRLab, USA), K. K. Das (California Institute of Technology, USA), A. Gal-Yam (Weizmann Institute of Science, Israel), D. Gruen (Ludwig Maximilian University of Munich, Germany/Excellence Cluster ORIGINS, Germany), S. Heathcote (NSF NOIRLab, USA), L. Hu (Carnegie Mellon University, USA), V. Karambelkar (California Institute of Technology, USA), M. Kasliwal (California Institute of Technology, USA), K. Labrie (NSF NOIRLab, USA), D. Pasham (Eureka Scientific, USA/The George Washington University, USA), A. Riffeser, M. Schmidt, K. Sharma, S. Wilke (Ludwig Maximilian University of Munich, Germany), & W. Zang (Center for Astrophysics | Harvard & Smithsonian, USA).

NSF NOIRLab, the U.S. National Science Foundation center for ground-based optical-infrared astronomy, operates the International Gemini Observatory (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), NSF Kitt Peak National Observatory (KPNO), NSF Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and NSF–DOE Vera C. Rubin Observatory (in cooperation with DOE’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. 

The scientific community is honored to have the opportunity to conduct astronomical research on I’oligam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawai‘i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence of I’oligam Du’ag to the Tohono O’odham Nation, and Maunakea to the Kanaka Maoli (Native Hawaiians) community.

The Dark Energy Camera was designed specifically for the Dark Energy Survey (DES). It was funded by the U.S. Department of Energy (DOE) and was built and tested at DOE's Fermilab.

Links

• Read the paper: Optical/infrared observations of the extraordinary GRB 250702B: a highly obscured afterglow in a massive galaxy consistent with multiple possible progenitors
• NASA press release
• Photos of the Víctor M. Blanco 4-meter Telescope
• Videos of the Víctor M. Blanco 4-meter Telescope
• Photos of DECam
• Photos of the Gemini North telescope
• Videos of the Gemini North telescope
• Photos of the Gemini South telescope
• Videos of the Gemini South telescope
• Check out other NOIRLab Science Releases

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