An Upper Limit on the Expansion Velocity of Gamma-Ray Burst Candidate SN 2001em

We report on VLBI observations of the Type I b/c supernova SN 2001em, 3 years after the explosion. It has been suggested that SN 2001em might be a jet-driven gamma-ray burst (GRB), with the jet oriented far from the line of sight so that the GRB would not be visible from Earth. To test this conjecture, we determined the size of SN 2001em. It is only marginally resolved at our resolution of ~0.9 mas. The 3 σ upper limit on the major axis angular size of the radio source was 0.59 mas (FWHM of an elliptical Gaussian), corresponding to a one-sided apparent expansion velocity of 70,000 km s-1 for a distance of 80 Mpc. No low-brightness jet was seen in our image to a level of 4% of the peak brightness. If we assume instead a spherical shell geometry typical of a supernova, we find the angular radius of SN 2001em was 0.17 mas, implying an isotropic expansion velocity of 20,000 km s-1, which is comparable to the expansion velocities of supernova shells. Our observations are therefore not consistent with a relativistically expanding radio source in SN 2001em but are instead consistent with a supernova shell origin of SN 2001's radio emission.