We report on VLBI , as well as VLA radio observations of the Type Ib/c supernova 2009bb .
The high radio luminosity of this supernova seems to require relativistic outflow , implying that the early radio emission was “ engine-driven ” , that is driven by collimated outflow from a compact object , even though no gamma-ray emission was seen .
The radio light curve shows a general decline , with a “ bump ” near t = 52 d , seen most prominently at 5 GHz .
The lightcurve bump could be either engine-driven , or it might represent the turn-on of the normal radio emission from a supernova , driven by interaction with the CSM rather than by the engine .
We undertook VLBI observations to resolve SN 2009bb ’ s relativistic outflow .
Our observations constrain the angular outer radius at an age of 85 d to be < 0.64 mas , corresponding to < 4 \mbox { $ \times 10 ^ { 17 } $ } cm and an average apparent expansion speed of < 1.74 c .
This result is consistent with the moderately relativistic ejecta speeds implied by the radio luminosity and spectrum .