SN 2001em: Not so fast

SN 2001em, originally classified as type Ib/c, is a peculiar supernova. It was observed in the radio about two years after its optical detection, showing a rising radio flux with an optically thin spectral slope; it also displayed a large X-ray luminosity (∼10⁴¹ erg s^-1). Thus, it was suspected to harbor a decelerating (by then, mildly) relativistic jet pointing away from us. About three years after its discovery, the optical spectrum of SN 2001em showed a broad Hα 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 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^-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, 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.