Limit to the radio emission from a putative central compact source in SN1993J

TL;DR

This study uses decade-long VLBI observations of supernova SN1993J to search for a central compact source, setting upper limits on its radio emission and improving constraints on potential black hole or pulsar presence.

Contribution

It provides the first comprehensive upper limits on the radio emission from a putative central compact object in SN1993J using stacking of VLBI images.

Findings

Upper limit of ~102 μJy at 5 GHz for a central source.

Ejecta opacity correction increases the limit to ~192 μJy.

No detection of a compact central source within the sensitivity limits.

Abstract

SN1993J in M81 is the best studied young radio-luminous supernova in the Northern Hemisphere. We recently reported results from the analysis of a complete set of VLBI observations of this supernova at 1.7, 2.3, 5.0, and 8.4 GHz, covering a time baseline of more than one decade. Those reported results were focused on the kinematics of the expanding shock, the particulars of its evolving non-thermal emission, the density profile of the circumstellar medium, and the evolving free-free opacity by the supernova ejecta. In the present paper, we complete our analysis by performing a search for any possible signal from a compact source (i.e., a stellar-mass black hole or a young pulsar nebula) at the center of the expanding shell. We have performed a stacking of all our VLBI images at each frequency, after subtraction of our best-fit shell model at each epoch, and measured the peak intensity in the stacked residual image. Given the large amount of available global VLBI observations, the stacking of all the residual images allows us to put upper limits to the eventual emission of a putative compact central source at the level of $\sim102$ $\mu$Jy at 5 GHz (or, more conservatively, $\sim192$ $\mu$Jy, if we make a further correction for the ejecta opacity) and somewhat larger at other wavelengths.