Late emission from the type Ib/c SN 2001em: Overtaking the hydrogen envelope

The Type Ib/c supernova SN2001em was observed to have strong radio, X-ray, and H alpha emission at an age of similar to 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 (similar to 3 M-circle dot) circumstellar shell at a distance of similar to 7 x 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 similar to(2-10); 10(-3) M-circle dot yr(-1) at similar to(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.