Late Emission from the Type Ib/c SN 2001em: Overtaking the Hydrogen Envelope

The Type Ib/c supernova SN 2001em was observed to have strong radio, X-ray, and Hα emission at an age of ~2.5 yr. Although the radio and X-ray emission have been attributed to an off-axis gamma-ray burst, we model the emission as the interaction of normal SN Ib/c ejecta with a dense, massive (~3 M ☉ ) circumstellar shell at a distance of ~7 × 10 16 cm. We investigate two models, in which the circumstellar shell has or has not been overtaken by the forward shock at the time of the X-ray observation. The circumstellar shell was presumably formed by vigorous mass loss with a rate of ~(2-10) × 10 -3 M ☉ yr -1 at ~(1-2) × 10 3 yr prior to the supernova explosion. The hydrogen envelope was completely lost and subsequently was swept up and accelerated by the fast wind of the presupernova star up to a velocity of 30-50 km s -1 . Although interaction with the shell can explain most of the late emission properties of SN 2001em, we need to invoke clumping of the gas to explain the low absorption at X-ray and radio wavelengths.