23 GHz VLBI Observations of SN 2008ax

TL;DR

This paper reports VLBI observations of supernova SN 2008ax at 23 GHz, detecting its radio emission and estimating its expansion velocity, which exceeds optical measurements, with implications for understanding supernova dynamics.

Contribution

First VLBI detection of SN 2008ax at 23 GHz, providing high-resolution imaging and velocity estimates that challenge optical-based velocity measurements.

Findings

Detected supernova with marginal VLBI imaging.

Estimated supernova expansion velocities are significantly higher than optical estimates.

Flux density measurements are consistent with VLA observations.

Abstract

We report on phase-referenced 23 GHz Very-Long-Baseline-Interferometry (VLBI) observations of the type IIb supernova SN 2008ax, made with the Very Long Baseline Array (VLBA) on 2 April 2008 (33 days after explosion). These observations resulted in a marginal detection of the supernova. The total flux density recovered from our VLBI image is 0.8$\pm$0.3 mJy (one standard deviation). As it appears, the structure may be interpreted as either a core-jet or a double source. However, the supernova structure could be somewhat confused with a possible close by noise peak. In such a case, the recovered flux density would decrease to 0.48$\pm$0.12 mJy, compatible with the flux densities measured with the VLA at epochs close in time to our VLBI observations. The lowest average expansion velocities derived from our observations are $(1.90 \pm 0.30) \times 10^5$ km s$^{-1}$ (case of a double source) and $(5.2 \pm 1.3) \times 10^4$ km s$^{-1}$ (taking the weaker source component as a spurious, close by, noise peak, which is the more likely interpretation). These velocities are 7.3 and 2 times higher, respectively, than the maximum ejecta velocity inferred from optical-line observations.