VLBI observations of supernova PTF11qcj: Direct constraints on the size of the radio ejecta

We present High Sensitivity Array (HSA) and enhanced Multi Element Remotely Linked Interferometer Network (eMERLIN) observations of the radio-loud broad-lined type Ic supernova PTF11qcj obtained $\sim7.5$ 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 H$\alpha$ spectral features. The Very Long Baseline Interferometry (VLBI) observations presented here show that the PTF11qcj GHz radio ejecta remains marginally resolved at the sub-milliarcsecond level at $\approx 7.5$ 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 light curve 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 in the radio may strengthen further this conclusion.