VLBI Observations of the Nearby Type IIb Supernova 2011dh

TL;DR

This study presents VLBI radio observations of supernova 2011dh at two epochs, measuring its expansion velocity and radius evolution, confirming synchrotron self-absorption models and providing insights into the supernova's shock dynamics.

Contribution

First direct VLBI measurement of supernova 2011dh's radius and expansion velocity, validating spectral modeling methods for supernova radio emission.

Findings

Measured angular radius of 0.25 mas at 179 days

Expansion velocity of 21000 km/s with uncertainty

Radius evolves as R ∝ t^{0.92}

Abstract

We report on phase-referenced VLBI radio observations of the Type IIb supernova 2011dh, at times t = 83 days and 179 days after the explosion and at frequencies, respectively, of 22.2 and 8.4 GHz. We detected SN 2011dh at both epochs. At the first epoch only an upper limit on SN 2011dh's angular size was obtained, but at the second epoch, we determine the angular radius SN 2011dh's radio emission to be 0.25 +- 0.08 mas by fitting a spherical shell model directly to the visibility measurements. At a distance of 8.4 Mpc this angular radius corresponds to a time-averaged (since t=0) expansion velocity of the forward shock of 21000 +- 7000 km/s. Our measured values of the radius of the emission region are in excellent agreement with those derived from fitting synchrotron self-absorbed models to the radio spectral energy distribution, providing strong confirmation for the latter method of estimating the radius. We find that SN 2011dh's radius evolves in a power-law fashion, with R proportional to t^(0.92 +- 0.10).