afterglows

An afterglow is the emission that follows a transient, short-duration flash, lasting from hours to weeks and spanning a broad range of wavelengths. The term is widely used in high-energy astrophysics to describe the fading radiation that follows gamma-ray bursts, where the prompt gamma-ray or hard X-ray emission is quickly supplanted by X-ray, optical, infrared, and radio emission. Afterglows are also observed in other explosive transients such as supernovae and kilonovae, though the GRB afterglow is the most studied form.

In the prevailing fireball or external shock model, the afterglow arises when an ultrarelativistic jet ejected by the explosion plows into the surrounding medium. The shock accelerates electrons, which radiate via synchrotron processes, producing a continuous spectrum that shifts to lower frequencies as the burst decelerates. The light curve typically follows a power-law decay in time, and breaks in the slope can reveal jet opening angles, changes in the circumburst density, or cooling dynamics.

Observationally, afterglows are tracked across X-ray, optical/near-infrared, and radio bands. Early X-ray measurements with satellites such as Swift constrain energetics and environments; optical observations enable redshift measurements and host-galaxy studies; radio data probe the slowest-evolving component and the jet structure.

Variations include flares, plateaus, and jet breaks, reflecting continued energy injection or structured outflows. The presence and brightness of afterglows depend on factors such as total energy, ambient density, and line-of-sight extinction. Afterglows provide key diagnostics of explosive physics, jet physics, and interstellar environments.