23 GHz VLBI Observations of SN 2008ax
  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.
