Hypernovae, collapsars, and gamma-ray bursts

Cosmic gamma-rag bursts now appropriately hold the distinction of being the "largest explosions in the universe". Their afterglows are often brighter than supernovae, thus often referred to as "hypernovae". Their kinetic ener gies may also be greater, or at least highly collimated, and require a new sour ce of energy. Recent photometric and spectroscopic observations of the afterglow emission have provided a major breakthrough in our understanding of these powerful explosions. The data place at least some bursts at large distances and in association with faint host galaxies. But what is (or are ) the underlying cause(s) of these violent events'! The answer to this ques tion remains uncertain, but several theoretical arguments point towards the creation of hyperaccreting black holes with accretion rates from 10(-4) to 10 solar masses per second, whose accretion disks produce narrow jets of r elativistically expanding plasma. We review the basic concepts of one of th ese models, the "collapsar model".