Hypernovae, collapsars, and gamma-ray bursts

Abstract

Cosmic gamma-ray 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 energies may also be greater, or at least highly collimated, and require a new source 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 question 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 relativistically expanding plasma. We review the basic concepts of one of these models, the “collapsar model”.