The supernova-gamma-ray burst connection

SN 1998bw and its corresponding relativistically expanding radio source are coincident with the gamma-ray burst source GRB 980425. We searched for other possible supernova-gamma-ray burst associations among 101 recent Type Ia and 17 Type Ib/c supernovae (SNe) for which the dates of optical maximum are relatively well known. We show that Type Ia SNe can be excluded as the source of gamma-ray bursts (GRBs) at a high significance level (greater than 4 sigma) and that the current data are consistent with Type Ib/c SNe as GRB producers. If the Type Ib/c event SN 1998bw does not represent a special class of GRBs, but is representative of the general mechanism for GRBs, it would imply GRB energies that are some fraction of an SN energy. This in turn demands strong collimation into perhaps 10(-3) sr to explain GRBs at higher redshifts. Such collimation requires a high rate of occurrence, perhaps consistent with an SN rate. The collimated flow may be generated by core collapse to produce rotating, magnetized neutron stars. Having lost most or all of their hydrogen and helium envelope before explosion, Type Ib/c SNe are particularly amenable to SN-GRB associations. Asymmetries in Line profiles and spectropolarimetry of Types II and Ib/c SNe, pulsar runaway velocities, soft gamma-ray repeaters, and GRBs may be associated phenomena.