We report on 8.4 GHz Very Long Baseline Interferometry ( VLBI ) observations of the type II-P supernova SN 2004et 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 8.4 GHz of ( 4.45 \pm 0.3 ) \times 10 ^ { 25 } erg s ^ { -1 } Hz ^ { -1 } and a brightness temperature of ( 1.3 \pm 0.3 ) \times 10 ^ { 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 .