Fermi spots one of the brightest gamma-ray bursts in 20 years

On March 10, 2026, NASA’s Fermi Gamma-ray Space Telescope detected a cosmic explosion designated GRB 260310A that ranks among the brightest gamma-ray bursts in the last 20 years. The explosion channeled roughly one billion years’ worth of solar energy into a narrow, pencil-like jet. The initial gamma-ray flash lasted under a minute and at first appeared feeble, but the afterglow quickly intensified to outshine its host galaxy within hours, with X-ray emissions lingering for months afterward. The findings, detailed in a study recently published in The Astrophysical Journal Letters, were detailed by Roberto Ricci, a radioastronomer at Italy’s National Institute of Astrophysics, with study co-author Yu-Han Yang, an astronomer who initially suspected X-ray contamination before concluding the brightening came from the afterglow.

The immense apparent brightness of GRB 260310A stems from how gamma-ray bursts concentrate their energy. Astronomers often describe a burst’s total energy as the isotropic-equivalent energy, the amount it would release if the explosion were perfectly spherical, but in reality the energy is funneled through a tightly collimated relativistic jet, so when the jet points directly at Earth, the burst appears extraordinarily bright. Astronomer Yu-Han Yang compared this to looking directly at a flashlight: the light is strongest from that view but weaker when seen from the side. For GRB 260310A, early signs indicated the jet was pointed away from Earth, yet the burst still ranked in the brightest one percent of recorded events in visible and X-ray light and in the top five percent for radio brightness, and Yang noted that standard GRB models are built around head-on detections, making this off-axis viewing angle an exception that gives researchers a chance to look beyond the glare and probe the jet’s structure from the outer edges toward the inner core.

The narrowness of the jet also dictates how the afterglow evolves, since afterglows are produced when the relativistic jet slams into cosmic gas, creating a shock that radiates across the spectrum and emits radiation across visible light, X-rays, and radio waves, giving astronomers multiple bands to track the fading emission. In the case of GRB 260310A, the afterglow brightened within hours to briefly outshine its host galaxy, then faded, but a few weeks later X-ray emissions brightened again and remained observable for months; study co-author Yu-Han Yang initially suspected another X-ray source was contaminating the observations before concluding the brightening came from the afterglow. Astronomers hypothesize that the burst featured a narrow, high-energy core surrounded by wider, slower outflows, so the early afterglow came from the jet’s wings while the later X-ray brightening resulted from the thin core, and the extended X-ray emission gave astronomers a prolonged window to study the jet’s energy dissipation.

Tracking a GRB’s afterglow requires a network of telescopes across the electromagnetic spectrum, and for GRB 260310A astronomers used an international array of observatories to identify the supernova responsible and monitor the fading emission. A detailed example of such a campaign comes from the study of GRB 220706A, an ultra-long burst detected on July 6, 2022; ultra-long GRBs are rare events whose central engines keep running longer than usual, and only a few confirmed cases are known, making this study particularly valuable. A team led by Benjamin P. Gompertz of the University of Birmingham gathered X-ray data from the Swift satellite, NICER, and the Chandra X-ray Observatory, optical and infrared images from four ground-based telescopes, and radio and millimeter observations from the NOEMA array and the Jansky Very Large Array, with follow-up beginning roughly 100 seconds after the burst and continuing until October 5, 2023; among 549 bursts examined, GRB 220706A ranked eighth for length with an opening burst of roughly 15.6 hours, and the X-ray data showed repeated flares lasting about 51 days, which after correcting for the redshift of z = 0.8577 meant the flaring lasted 27.25 days in the burst’s rest frame – about 20.6 days beyond the earlier record held by GRB 210204A.

The study, posted to arXiv on Sept. 18, also noted that GRB 220706A is classed as a dark burst because its optical light was far fainter than expected from its X-rays, likely due to dust in the host galaxy, and seventeen days after the burst additional optical light appeared in line with a supernova component that could be a superluminous supernova if confirmed – the first time a gamma-ray burst and a superluminous supernova might have occurred together.

Off-axis bursts like GRB 260310A offer a rare opportunity to probe the internal structure of relativistic jets, since the jet is not pointed directly at Earth astronomers can study its outer edges before peering into the high-energy core, and Yang explained that the side-on view allows researchers to observe how the jet’s structure changes from wider, slower outflows to a thin, fast core; the study’s authors hypothesize that the early afterglow came from the jet’s wings while the later X-ray brightening originated from the core itself, a model that would unify the burst’s seemingly contradictory behaviors into a single picture of a structured jet. Roberto Ricci, a radioastronomer at Italy’s National Institute of Astrophysics, said that individual pieces of the burst could be explained, but combining them into one model was far more difficult. This insight into jet structure also connects to the broader question of how stars die, since gamma-ray bursts are typically linked to the collapse of massive stars that form black holes or neutron stars, or to the merger of neutron stars, and GRB 260310A was traced to a supernova, reinforcing this link. Meanwhile, GRB 220706A challenged existing models by keeping its central engine active for 27 days in its rest frame, and researchers considered several mechanisms for the prolonged activity: accretion onto a black hole from a large, extended star, which strained standard models; a magnetar engine, which fits some other ultra-long GRBs but would need much parameter adjustment here, with newborn magnetars not expected to have fully formed crusts at the relevant timescales; and a star being torn apart by a black hole, which fits the burst’s central location geometrically but whose X-ray and optical behavior resembled an ordinary GRB more than a tidal disruption; in the end, the researchers preferred a massive-star collapse accompanied by a supernova, citing a previously confirmed case as the closest match, though none of the proposed mechanisms accounts for every detail and the optical excess attributed to a supernova might instead be an unusual counterpart to continuing X-ray flares.

Events like GRB 260310A are rare, but they are among the most informative. Roberto Ricci, a radioastronomer at Italy’s National Institute of Astrophysics, said, “We see hundreds of GRBs every year, but the oddballs are the most informative ones.” The team hopes that the recently completed Vera Rubin Observatory will detect more such off-axis bursts, providing a larger sample to test theories of jet formation and evolution. The GRB 220706A study, led by Benjamin P. Gompertz and colleagues, was published in 2026 with DOI 10.48550/arxiv.2609.22426.