8.4GHz VLBI observations of SN2004et in NGC6946

TL;DR

This paper presents high-resolution VLBI observations of supernova SN2004et, revealing asymmetries and providing insights into its expansion velocity and emission mechanisms, challenging some existing models of supernova radio emission.

Contribution

First VLBI imaging of SN2004et at 8.4GHz, showing asymmetry and constraining expansion velocity, with implications for supernova emission models.

Findings

VLBI image shows asymmetry in SN2004et

Estimated expansion velocity exceeds synchrotron self-absorption predictions

Supports a two hot spots emission model on an expanding shell

Abstract

We report on 8.4GHz Very Long Baseline Interferometry (VLBI) observations of the type II-P supernova SN2004et in the spiral galaxy NGC 6946, made on 20 February 2005 (151 days after explosion). The Very Large Array (VLA) flux density was 1.23$\pm$0.07 mJy, corresponding to an isotropic luminosity at 8.4GHz of (4.45$\pm$0.3)$\times10^{25}$ erg s$^{-1}$ Hz$^{-1}$ and a brightness temperature of (1.3$\pm$0.3)$\times10^{8}$ K. We also provide an improved source position, accurate to about 0.5 mas in each coordinate. The VLBI image shows a clear asymmetry. From model fitting of the size of the radio emission, we estimate a minimum expansion velocity of 15,700$\pm$2,000 km s$^{-1}$. This velocity is more than twice the expected mean expansion velocity estimated from a synchrotron self-absorbed emission model, thus suggesting that synchrotron self-absorption is not relevant for this supernova. With the benefit of an optical spectrum obtained 12 days after explosion, we favor an emission model which consists of two hot spots on an underlying expanding shell of width comparable to that of SN 1993J.