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

TL;DR

This study uses VLBI observations to measure the size of the radio ejecta of supernova PTF11qcj, providing constraints on its expansion and interaction with the surrounding medium, and suggesting a non-relativistic expansion scenario.

Contribution

First direct VLBI measurements of supernova PTF11qcj's radio ejecta size, constraining its expansion velocity and interaction with circumstellar material.

Findings

Ejecta remains marginally resolved at 7.5 years post-explosion

Ejecta expansion is non-relativistic

Radio light curve peaks are due to circumstellar interaction

Abstract

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.