gamma-ray burst afterglow
A gamma-ray burst itself is over in seconds, a flash too brief and too narrowly located to study in detail. But the burst is not the end of the story. After the gamma rays fade, the same spot on the sky keeps glowing — first in X-rays, then ultraviolet and visible light, then radio — slowly fading over hours, days, and weeks. This lingering glow is the afterglow, and it is where most of what we know about gamma-ray bursts actually comes from.
The afterglow comes from the burst's jet running into the thin gas around it. The jet, moving at almost the speed of light, drives a shock into the surrounding material; that shock accelerates electrons (by Fermi acceleration) and they radiate synchrotron light across the spectrum. As the jet slows and the shock weakens, the glow fades and shifts from high to low energies — which is why an afterglow first seen in X-rays is later best caught in radio. The afterglow is much dimmer than the burst, but it lasts long enough for telescopes around the world to study it.
Afterglows transformed gamma-ray burst science from the late 1990s on. Because they linger, they let astronomers pin down a burst's precise location, identify its home galaxy, and measure its distance — proving that bursts lie billions of light-years away and are therefore staggeringly powerful. The afterglow's slow fade and changing colors also reveal the jet's structure and the density of its surroundings, and a sudden steepening in the fade can betray the edge of the jet, helping measure how tightly the burst was beamed.
In 1997, astronomers caught a fading X-ray afterglow at a burst's position, then a visible-light glow, and traced it to a faint distant galaxy. Measuring that galaxy's redshift finally settled a decades-old debate: gamma-ray bursts are cosmological, lying billions of light-years away, not nearby. The afterglow, not the burst, gave the answer.
The slow, fading glow after the burst — in X-ray, light, then radio — that reveals distance and jet.
The afterglow is not a second explosion — it is the same jet still glowing as it plows into surrounding gas, shifting to ever lower energies as it slows. Catching it fast matters: the visible afterglow can fade by a factor of a hundred within a single day.