Gamma Ray Bursts and Their Afterglows

Gamma-Ray Bursts are extreme astrophysical events, which emit the bulk of their energy as photons in the 0.1 – 1.0 MeV range, and whose durations span milliseconds to tens of minutes. They are formed in extreme relativistic outflows with Lorentz factors of hundreds, and reside at cosmological distances. They are followed by X-ray, optical and radio afterglows which can be observed for over a year after the event. Observations of afterglows showed that the emission is from jets, and when corrected for this geometry the energies of GRBs appear to cluster around 5 × 10^50 erg — very comparable to that of supernovae. Evidence in the last several years shows that a significant fraction of long GRBs are related to a peculiar type of supernova explosions. These supernovae most likely mark the birth events of stellar mass black holes as the final products of the evolution of very massive stars. Short bursts are still somewhat mysterious, but it is known that some of them are produced by an old population of stars. Neutron star merger is a leading candidate as the progenitor of short GRBs.