Constraints on off-axis gamma-ray burst jets in Type Ibc supernovae from late-time radio observations

It has been suggested that the peculiar properties of the luminous Type Ic supernova SN 1998bw and its low-energy gamma-ray burst GRB 980425 may be understood if they originated in a standard gamma-ray burst explosion viewed far from the axis of the relativistic jet. In this scenario, strong radio emission is predicted from the jet on a timescale 1-10 yr after the explosion as it decelerates and spreads into our line of sight. To test this hypothesis, we have carried out late-time radio observations of SN 1998bw at t = 5.6 yr, yielding upper limits that are consistent with the continued fading of the supernova. We find these limits to be consistent with an off-axis jet only if the progenitor mass-loss rate is (M)over dot less than or similar to 4 x 10(-7) M-circle dot yr(-1) (for a wind velocity v(w) = 1000 km s(-1)) or the fraction of the shock energy in magnetic fields is epsilon(B) less than or similar to 10(-3). These values are low relative to those inferred for cosmological gamma-ray bursts. We combine the SN 1998bw measurements with existing observations for a sample of 15 local Type Ibc supernovae to estimate that at most 6% produce collimated, relativistic outflows.