SN 2001em: No Jet-Driven Gamma Ray Burst Event

TL;DR

This study used VLBI and VLA observations to show that supernova SN 2001em is expanding non-relativistically five years after explosion, contradicting earlier suggestions of a jet-driven gamma-ray burst.

Contribution

The paper provides the first direct VLBI imaging evidence that SN 2001em's expansion is non-relativistic, challenging previous jet-driven GRB hypotheses.

Findings

SN 2001em remains unresolved five years post-explosion.

Expansion velocity is consistent with non-relativistic supernova ejecta.

Radio flux density shows a power-law decay since 2003.

Abstract

We report on our second-epoch VLBI and VLA observations of the Type Ib/c supernova 2001em, five years after the explosion. It was suggested that SN 2001em might be a jet-driven gamma ray burst (GRB), 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 five years after the explosion. For a distance of 83 Mpc (H_0 = 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 non-relativistic 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, non-relativistic 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 lightcurve in ~2003.