A hypernova (alternatively called a collapsar) is a very energetic supernova thought to result from an extreme core-collapse scenario.

In this case a massive star (>30 solar masses) collapses to form a rotating black hole emitting twin energetic jets and surrounded by an accretion disk. This collapse happens so quickly that the outer parts of the star are unaware of what has taken place, and the star is subsequently exploded by vigourous winds of newly-formed 56Ni blowing off the accretion disk, and shock waves produced as the jets plough through the stellar material. The hypernova, whose luminosity is powered by the radioactive decay of 56Ni, is the result of the explosion of the star.

This model for the formation of a hypernova also predicts that these objects should be accompanied by a gamma ray burst (GRB). Although the mechanism to form the gamma rays is still a matter of debate, it is thought that they are produced through internal collisions within the jet itself. Whatever the actual mechanism, the gamma rays are beamed into a narrow cone along the direction of motion of the jet, and are visible to us only if the jet is pointed in our direction. Astronomers estimate that for every GRB we observe, there are several hundreds more we don’t see, those which are oriented in directions away from us.

Conclusive evidence for this hypernova-GRB connection was obtained only recently. Although there were many cases where the light curve of the prototype hypernova, SN1998bw, could be fitted to the light curve decay of gamma ray burst optical transients associated with GRBs, it was not until astronomers clearly observed the spectrum of a hypernova within the spectrum of an optical transient that the connection was firmly established.

Study Astronomy Online at Swinburne University
All material is © Swinburne University of Technology except where indicated.