2006ApJ...641.1051C

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 ∼7x10¹⁶ 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)x10^–3 M_☉/yr at ~(1-2)x10³ 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. 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.