SN 2001em: Not so Fast

TL;DR

SN 2001em, initially classified as Type Ib/c, exhibits late-time radio and X-ray emissions indicative of dense circumstellar interaction, but VLBI data show no evidence of a relativistic jet, refining understanding of its explosion dynamics.

Contribution

This study provides new VLBI constraints on SN 2001em's proper motion and expansion velocity, confirming the absence of a relativistic jet and enhancing knowledge of its late-time evolution.

Findings

Proper motion constrained to ~23,000 km/s with large uncertainty

Upper limit on expansion velocity is 6000 km/s

Radio flux decays as t^{-1.23} from 2.7 years post-explosion

Abstract

SN 2001em is a peculiar supernova, originally classified as Type Ib/c. About two years after the SN it was detected in the radio, showing a rising radio flux with an optically thin spectral slope, and it also displayed a large X-ray luminosity (~10^{41} erg/s). Thus it was suspected to harbor a decelerating (by then, mildly) relativistic jet pointing away from us. About 3 years after its discovery the optical spectrum of SN 2001em showed a broad H-alpha line, and it was therefore reclassified as Type IIn. Here we constrain its proper motion and expansion velocity by analyzing four epochs of VLBI observations, extending out to 5.4 years after the SN. The supernova is still unresolved 5.4 years after the explosion. For the proper motion we obtain (23,000 +/- 30,000) km/s while our 2-sigma upper limit on the expansion velocity is 6000 km/s. These limits are somewhat tighter than those derived by Bietenholz & Bartel, and confirm their conclusion that late time emission from SN 2001em, a few years after the explosion, is not driven by a relativistic jet. VLA observations of the radio flux density, at 8.46 GHz, show a decay as t^{-1.23 +/- 0.40} starting ~2.7 years after the SN. Collectively, the observations suggest interaction of the SN ejecta with a very dense circumstellar medium, though the implied opacity constraints still present a challenge.