Radio limits on off-axis GRB afterglows and VLBI observations of SN 2003gk

We report on a survey for late-time radio emission from 59 Type I b/c supernovae. Supernova of Type I b/c have been associated with long-duration gamma-ray bursts (GRBs), and it is expected that an “off-axis” burst (i.e. whose relativistic jet points away from us) would produce radio-emission at late times even in the absence of significant prompt gamma-ray emission. Of the supernovae in our sample, only SN 2003gk was detected in the radio with a flux density of 2260 ± 130µJy. We subsequently undertook VLBI observations of SN 2003gk at an age of �8 yr, which allowed us to determine its radius to be (2.4 ± 0.4) × 10 17 cm, or 94 ± 15 light days, which is not compatible with a relativistically expanding source expected in the case of an offaxis GRB jet. Instead, the average expansion speed of � 10 000 km s 1 is typical for non-relativistic core-collapse supernovae. We attribute the late-onset radio emission to interaction of the ejecta with a dense shell caused by episodic mass-loss from the progenitor. In addition, we present new calculations for the expected radio lightcurves from GRB jets at various angles to the line of sight, and compare these to our observed limits on the flux densities of the remainder of our sample of Type I b/c SNe. We find that bright jet emission, similar to that for GRBs detected at cosmological distances, is incompatible with our observed limits, and that therefore we can say only a fraction even of broadlined Type I b/c SNe have such a jet regardless of its orientation. However, we also find that for a reasonable range of parameters, as might be representative of the actual population of GRB events rather than the detected bright ones, the radio emission from the GRB jets can be quite faint, and that present radio observations do not place strong constraints on the presence of jets Type I b/c SNe.