Earth outside jet core explains faint flash and bright afterglow

A weak gamma-ray flash can produce one of the brightest afterglows in two decades when Earth views the stellar jet from the side rather than straight down its core, an international team of astronomers found after GRB 260310A erupted on March 10, 2026.

The finding shows how Earth’s position outside the primary beam can make a powerful stellar jet look faint in gamma rays while its afterglow shines brightly. These explosions are easiest to detect when a jet points directly toward Earth because the jet appears brightest head-on. An off-axis line of sight keeps the initial flash faint, yet the afterglow can still brighten as the jet slows and its light spreads across wider angles.#

NASA’s Fermi Gamma-ray Space Telescope detected GRB 260310A on March 10, 2026, and the initial gamma-ray flash lasted roughly 53 seconds. The flash apparently released little energy, which initially drew astronomers’ attention before the afterglow observations changed the picture. The burst ultimately produced one of the brightest afterglows recorded in the past 20 years.#

In visible light, the afterglow increased gradually over hours and surpassed the brightness of its host galaxy before fading. This optical behavior contrasted with the modest prompt gamma-ray signal. Weeks after the initial flash, the X-ray glow brightened again and stayed visible for months, indicating prolonged high-energy emission.#

Three weeks after the burst, the X-ray afterglow brightened once more and stayed bright for over a week before fading. The brightening surprised the researchers. Yu-Han Yang initially suspected contamination from another X-ray source before realizing the signal came from the afterglow. Yang coordinated the X-ray monitoring, and the unexpected brightening prompted the team to explore a more complex jet shape.#

The BHianca group at the University of Rome Tor Vergata led the research team. The astronomers used a fleet of telescopes to follow the afterglow across X-rays and radio waves, coordinating observations from major facilities. The observing campaign combined 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. The multi-wavelength coverage let the team track the emission as it evolved.#

Separately, observations at Calar Alto Observatory revealed a supernova signature, confirming that the burst resulted from the death of a massive star. This link to a stellar collapse placed GRB 260310A among the bursts tied to dying massive stars. The supernova signature anchored the burst in the massive-star death channel.#

Researchers published the study in The Astrophysical Journal Letters. The paper carries the title An Extremely Bright Slow-rising Afterglow from an Off-axis Jet in GRB 260310A and appeared in the journal in 2026. Its DOI is 10.3847/2041-8213/aea990, and the University of Rome Tor Vergata provided the material.#

Gamma-ray bursts are brief flashes of gamma rays, the most energetic form of light known. Many gamma-ray bursts occur when a massive star collapses and launches jets of matter outward at nearly the speed of light. When a jet plows into surrounding gas, it creates an afterglow visible in X-rays, visible light and radio waves.#

These explosions are easiest to detect when a jet points directly toward Earth because the jet appears brightest head-on. This head-on bias means researchers calibrate most models using events seen head-on, according to postdoctoral scholar and lead author Yu-Han Yang, and this burst offered a view beyond that glare.#

In GRB 260310A, Earth sat outside the primary tightly focused beam, so the jet pointed away from our planet and its gamma-ray output appeared unremarkable. According to Yu-Han Yang, the burst showed telltale signatures of a jet pointing away from Earth, with unremarkable gamma-ray output compared to typical bursts. The mismatch between the faint flash and the bright afterglow came down to perspective.#

The team looked at the burst from a side angle. Once the jet decelerated, its light reached a broader span of angles. This off-axis perspective let the team probe the jet’s shape from the outer edges toward the inner core, revealing structure invisible to a head-on observer.#

In the team’s comparison samples, the observed X-ray and optical afterglow ranked among the brightest 1% of events, while its radio brightness placed in the top 5%. Those rankings showed the burst was intrinsically powerful even though its opening flash seemed weak. The event sat among the most energetic afterglows in the team’s comparison set.#

One possible explanation is that a jet featuring a narrow, energetic core surrounded by a broader, slower outflow powered the burst. In this picture, early afterglow radiation comes from the broader wings of the jet as seen from the side. Light from its narrow core becomes visible later as the flow decelerates, causing the X-ray afterglow brightening.#

Models suggest the explosion packed about a billion years of solar energy output into a pencil-thin beam. Yu-Han Yang noted that an unremarkable GRB could have resembled a monster explosion to an observer looking straight down the jet’s core. Such energy concentrated in a narrow beam means even a side view can reveal a powerful blast.#

Co-author Roberto Ricci led the radio observations and remarked that oddball bursts are the most informative among the hundreds of GRBs detected annually. “We see hundreds of GRBs every year, but the oddballs are the most informative ones,”#

As new generation sky surveys such as the one at the Vera C. Rubin Observatory ramp up, the team hopes to find more afterglows from powerful jets that never point toward Earth. Studying these hidden explosions would show how often dying stars launch jets. It would also clarify how these jets punch through the stars’ outer layers.#

Muskan Yadav, a graduate student who contributed to the study, concluded that gamma rays are not the most efficient route to spotting hidden jets, which astronomers find once in a blue moon. Upcoming surveys will pick up hundreds of hidden jets, but identifying them among millions of variable sources will not be easy.

The geometry of GRB 260310A illustrates how a faint flash can hide a powerful jet when Earth’s line of sight misses the tight core. The team’s work adds to the understanding of off-axis cosmic transients and stellar-jet structure, and it shows why viewing angle matters when judging an explosion’s true power. Future surveys may reveal more of these hidden explosions.#