A cosmic explosion that announced itself with a modest flash of gamma rays went on to produce one of the brightest afterglows astronomers have recorded in the past 20 years. An international team of astronomers now finds evidence that the surprising mismatch comes down to perspective: Earth was outside the most intensely beamed light from the explosion.
The event, called GRB 260310A, was detected by NASA's Fermi Gamma-ray Space Telescope on March 10, 2026. Its initial gamma-ray flash lasted about 53 seconds and caught the astronomers' attention for releasing apparently little energy. But the glow that followed told them a different story. In visible light, the afterglow brightened gradually over several hours, becoming brighter than the galaxy that hosted it, before fading away. Then, weeks after the initial flash, its X-ray glow brightened again and remained visible for months.
The team, led by the ERC-funded group BHianca at the University of Rome Tor Vergata, enlisted a fleet of telescopes to track the afterglow from X-rays to radio waves. The campaign brought together NASA's Chandra X-ray Observatory, ESA's XMM-Newton, the Einstein Probe mission, the Large Binocular Telescope, and the Karl G. Jansky Very Large Array. Observations at Calar Alto Observatory revealed the telltale signature of a supernova, confirming that the burst marked the death of a massive star. The study was published in The Astrophysical Journal Letters.
"We see hundreds of GRBs every year, but the oddballs are the most informative ones," says co-author Roberto Ricci, who led the radio observations. "We could explain individual features of this burst, but fitting them all into the same picture proved much harder."
Gamma-ray bursts are brief flashes of gamma rays, the most energetic form of light. Many occur when a massive star collapses, launching jets of matter that race outward at nearly the speed of light. As a jet plows into surrounding gas, it produces an afterglow that telescopes can follow in X-rays, visible light, and radio waves.
Astronomers most easily detect these explosions when a jet points directly toward Earth. "Much like a flashlight, the jet appears brightest when seen head-on. Most of our models are calibrated using events seen from that same perspective. This burst gave us a chance to look beyond that glare," explains Yu-Han Yang, a postdoctoral scholar and lead author of the study. According to Yang, GRB 260310A showed the telltale signatures of a jet pointing away from Earth. Its gamma-ray output was unremarkable. Yet in the team's comparison samples, its observed X-ray and optical afterglow ranked among the brightest 1%, and its radio brightness among the top 5%. "We looked at it from a side angle. As the jet slowed down, its light became visible over a wider range of angles, allowing us to probe the jet's shape, from its outer edges toward its inner core."
Three weeks after the initial burst, the X-ray afterglow brightened again and remained bright for over a week, before fading again. "That was unexpected. At first, I thought another X-ray source was contaminating our observations. But the brightening really came from the afterglow," recalls Yang, who coordinated the X-ray monitoring. "This prompted us to explore a more complex jet's shape."
A possible explanation is that the burst was powered by a jet with a narrow, energetic core surrounded by a broader, slower outflow. In this picture, the early afterglow radiation comes from the broader wings of the jet, while light from its narrow core becomes visible later causing the brightening of the X-ray afterglow.
"What appeared to us as an unremarkable GRB might have looked like a monster explosion to an observer looking straight down the jet's core," notes Yang. "Our models suggest that the explosion packed roughly a billion years of the Sun's energy output into a pencil-thin beam."
As a new generation of sky surveys, such as the one at the Vera Rubin Observatory, ramps up, the team hopes to find more of these afterglows from powerful jets that never point toward Earth. Studying this overlooked population of explosions would help astronomers determine how often dying stars launch such jets and how these jets punch through the stars' outer layers.
"Gamma rays are not the most efficient route to spot these hidden jets: we find them once in a blue moon," concludes Muskan Yadav, a graduate student who contributed to the study. "Upcoming surveys will pick up hundreds of them. Identifying them among millions of variable sources won't be easy, but it's a challenge I'm excited to take on."
