Central engines for gamma-ray bursts

What powers a gamma-ray burst (GRB)? We discuss here some properties of several currently favored models based on black hole accretion with emphasis on the collapsar - a rotating massive star whose iron core collapse produces a black hole. Depending on mass, rotation rate, and viewing angle, collapsars can explain a wide gambit of GRBs from faint events like GRB 980425, to bright ones like GRB 971214. Because of accretion disk instabilities, the Γ 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 pre-explosive stellar wind. Beaming for hard gamma-rays is about 1%, but mildly relativistic matter is ejected at larger angles. All collapsars produce Type Ib/c supernovae like SN 1998bw, but the converse is not true. Most Type Ib/c supernovae do not make GRBs.