A Faint Gamma-Ray Burst Left One of the Brightest Afterglows Ever Observed

A gamma-ray burst that flashed only weakly in gamma rays went on to produce one of the brightest afterglows ever observed, and the team that followed it argues the mismatch comes down to viewing angle: the jet was aimed away from Earth. The result was published Oct. 28, 2026, in The Astrophysical Journal Letters.

Yu-Han Yang of the University of Rome Tor Vergata and colleagues say the radio data in their paper provide a first test of reverse-shock models, which describe the wave that runs back into a jet's own material, in a geometry where the jet does not point at the observer.

The burst, cataloged as GRB 260310A, was a nearby long gamma-ray burst accompanied by a supernova, a broad-lined Type Ic in the paper's classification. Its prompt gamma-ray output was modest, about 3.5 × 10⁵⁰ ergs, and yet the authors report one of the brightest afterglows ever observed in X-ray, optical and radio light. The optical afterglow arrived late and rose slowly, peaking about a tenth of a day after the burst, while the X-ray afterglow brightened again later and was tracked for about 68 days with no sign of its spectrum changing.

The authors argue that the weak prompt flash, its hard peak energy and the delayed afterglow are "naturally explained by a GRB jet viewed off-axis," meaning a jet whose axis misses Earth.

A weak polarization of the radio light, measured 55 days after the burst, points to the forward shock, the outward-moving shock front, as the dominant source of the emission from radio through X-rays at that stage. If the later X-ray brightening instead marks a narrow jet core seen still further off-axis, the authors write, it would imply extreme luminosities and energetics for an observer looking straight down the jet.