2007ApJ...665L..47B

We report on our second-epoch VLBI and VLA observations of the Type Ib/c supernova 2001em, 5 years after the explosion. It was suggested that SN 2001em might be a jet-driven gamma-ray burst, with the jet oriented near the plane of the sky, which would entail relativistic expansion or motion. Our VLBI image shows that SN 2001em is still unresolved 5 years after the explosion. For a distance of 83 Mpc (H₀=70 km/s/Mpc), the nominal expansion velocity is 5800±10,000 km/s, and the proper motion is 33,000±34,000 km/s. Our values are inconsistent with either relativistic expansion or motion but are consistent with the nonrelativistic expansion speeds and small proper motions seen in other supernovae. In particular, these values are consistent with radio emission from SN 2001em being due to normal, nonrelativistic supernova ejecta interacting with the circumstellar medium. Our VLA observations show a power-law decay in flux density since the time of the peak in the 8.4 GHz radio light curve sometime in 2003.