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

We report on a Very Large Array survey for late-time radio emission from 59 supernovae (SNe) of the Type I b/c, which is associated with long-duration gamma-ray bursts (GRBs). An `off-axis' GRB burst (i.e. whose relativistic jet points away from us) is expected to have late-time radio emission even in the absence of significant prompt gamma-ray emission. From our sample, we detected only SN 2003gk with an 8.4 GHz flux density of 2260 ± 130 μJy. Our subsequent very long baseline interferometry (VLBI) observations of SN 2003gk, at an age of ∼8 yr, allowed us to determine its radius to be (2.4 ± 0.4) × 10¹⁷ cm, or 94 ± 15 light days. This radius rules out relativistic expansion as expected for an off-axis GRB jet, and instead suggests an expansion speed of ∼10 000 km s^-1 typical for non-relativistic core-collapse SNe. 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 light curves 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 SN sample. From this comparison, we can say that only a fraction of broadlined Type I b/c SNe have a radio-bright jet similar to those seen for GRB afterglows at cosmological distances. 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 at present, radio observations do not place strong constraints on off-axis GRB jets.