The Supernova-Gamma Ray Burst Connection

Supernovae 1998bw and its corresponding relativistically expanding radio source are coincident with the γ-ray burst source GRB 980425. We show that of six recent SN Ib/c for which an outburst epoch can be estimated with some reliability, four have radio outbursts and all are correlated in time and space with BATSE γ-ray bursts. The joint probability of all six correlations is 1.5×10. No such correlation exists for SN Ia and SN II. The γ-ray energy associated with the SN/GRB events is ∼ 10 − 10 ergs if emitted isotropically. Economy of hypotheses leads us to propose that all γ-ray bursts are associated with supernovae and that the γ-ray burst events have a quasi-isotropic component that cannot be observed at cosmological distances and a strongly collimated and Doppler-boosted component that can only be seen if looking nearly along the collimation axis. Such collimation requires a high rate of occurrence perhaps consistent with a supernova rate. The collimated flow may be generated by core collapse to produce rotating, magnetized neutron stars. All core collapse events may produce such jets, but only the ones that occur in supernovae with small or missing hydrogen envelopes, Type Ib or Ic, can propagate into the interstellar medium and yield a visible γ-ray burst. We suggest that asymmetries in line profiles and spectropolarimetry of SN II and SN Ib/c, pulsar runaway velocities, soft γ-ray repeaters and γ-ray bursts are associated phenomena. Subject headings: supernovae: general − gamma rays: bursts − pulsars:general − ISM: jets and outflows