VLBI Observations of supernova PTF11QCJ: Direct constraints on the size of the radio ejecta

e present High Sensitivity Array (HSA) and enhanced Multi Element Remotely Linked Interferometer Network (eMERLIN) observations of the radio-loud broad-lined type Ic supernova PTF11qcjobtained∼7.5 years after the explosion. Previous observations of this supernova at 5.5 yrs since explosion showed a double-peaked radio light curve accompanied by a detection in the X–rays, but no evidence for broad Hαspectral features. The Very Long Baseline Interferometry (VLBI) observations presented here show that the PTF11qcj GHz radio ejecta remains marginally resolved at thesub-milliarcsecond level at≈7.5 yrs after the explosion, pointing toward a non-relativistic expansion.Our VLBI observations thus favor a scenario in which the second peak of the PTF11qcj radio lightcurve is related to strong interaction of the supernova ejecta with a circumstellar medium of variable density, rather than to the emergence of an off–axis jet. Continued VLBI monitoring of PTF11qcj inthe radio may strengthen further this conclusion.