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We present observations of the recently discovered supernova 2008iz in M82with the VLBI High Sensitivity Array at 22 GHz, the Very Large Array atfrequencies of 1.4, 4.8, 8.4, 22 and 43 GHz, and the Chandra X-ray observatory.The supernova was clearly detected on two VLBI images, separated by 11 months.The source shows a ring-like morphology and expands with a velocity of ~23000km/s. The most likely explosion date is in mid February 2008. The measuredexpansion speed is a factor of ~2 higher than expected under the assumptionthat synchrotron self-absorption dominates the light curve at the peak,indicating that this absorption mechanism may not be important for the radioemission. We find no evidence for an asymmetric explosion. The VLA spectrumshows a broken power law, indicating that the source was still optically thickat 1.4 GHz in April 2009. Finally, we report upper limits on the X-ray emissionfrom SN 2008iz and a second radio transient recently discovered by MERLINobservations.

VLBI observations of SN 2008iz: I. Expansion velocity and limits on anisotropic expansion