Central engines for gamma-ray bursts

What pourers a gamma-ray burst (GRB)? We discuss here some properties of se veral currently favored models based on black hole accretion with emphasis on the collapsar - a rotating massive star whose iron core collapse produce s a black hole. Depending on mass; rotation rate, and viewing angle, collap sars can explain a wide gambit of GRBs from faint events like GRB 980425, t o bright ones like GRB 971214. Because of accretion disk instabilities. the Gamma in the jet may be rapidly time variable. The burst itself is made by a combination of internal shocks in the jet and external shocks with the p re-explosive stellar wind. Beaming for hard gamma-rays is about 1%, but mil dly relativistic matter is ejected at larger angles. All collapsars produce Type Ib/c supernovae like SN 1998bw, but the converse is not true. Most Ty pe Ib/c supernovae do not make GRBs.