Gamma-ray burst afterglows

The discovery of counterparts in X-ray and optical to radio wavelengths has revolutionized the study of gamma -ray bursts, until recently the most eni gmatic of astrophysical phenomena. We now know that gamma -ray bursts are t he biggest explosions in nature, caused by the ejection of ultrarelativisti c matter from a powerful energy source and its subsequent collision with it s environment. We have just begun to uncover a connection between supernova e and gamma -ray bursts, and are finally constraining the properties of the ultimate source of gamma -ray burst energy. We review here the observation s that have led to this breakthrough in the field; we describe the basic th eory of the fireball model and discuss the theoretical understanding that h as been gained from interpreting the new wealth of data on gamma -ray burst s.