SN 2001em: Evidence for a dense circumstellar environment

SN 2001em is a peculiar supernova, originally classified as Type Ib/c. About two years after the occurrence of 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 (∼ 10⁴¹ erg s^{− 1}). About 3 years after its discovery the optical spectrum of SN 2001em showed a broad Hα line, and it was therefore reclassified as Type IIn. We constrained its proper motion and expansion velocity by analyzing four epochs of VLBI observations, extending out to 5.4 years after the SN. The SN is still unresolved 5.4 years after the explosion. For the proper motion we obtain (23,000 ±30,000) km s^{− 1} while our 2σ upper limit on the expansion velocity is 6000 km s^{− 1}. These limits are somewhat tighter than those derived by Bietenholz & Bartel. 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.