Gamma-Ray Bursts: Afterglows and Central Engines

Gamma-ray bursts (GRBs) are most intense transient gamma-ray events in the sky when they are on together with the strong evidences (i.e. the isotropic and inhomogeneous distribution of GRBs detected by BASTE) that they are located at cosmological distances, which make them the most energetic events ever known. For example, the observed radiation energies of some GRBs are equivalent to convert more than one solar mass energy into radiation completely. This is thousand times stronger than that of supernova explosion. Unconventional energy mechanisms and extremely high conversion efficiency for these mysterious events are required. The discovery of host galaxies and association with supernovae in the cosmological distances by the recently launched satellite of BeppoSAX and ground based radio and optical telescopes in GRB afterglow provides further support to the cosmological origin of GRBs and put strong constraints on central engines of GRBs. It is the aim of this article to review the possible central engines, energy mechanisms, dynamical and spectral evolution of GRBs, especially focusing on the afterglows in multi-wavebands. Subject headings: gamma-rays: bursts — shock waves — ISM: jets and outflows — radiation mechanisms: non-thermal