The Case for a Misaligned Relativistic Jet from SN 2001em

SN 2001em, identified as a Type Ic supernova, has recently been detected in the radio $\sim$ 2 yr after the explosion. The relatively high radio luminosity at such a late time might arise from a relativistic jet viewed substantially off-axis that becomes visible only when it turns mildly relativistic and its emission is no longer strongly beamed away from us. Alternatively, the radio emission might originate from the interaction of the SN shell with the circumstellar medium. We find that the latter scenario is hard to reconcile with the observed rapid rise in the flux and optically thin spectrum, $F_\nu\propto\nu^{-0.36\pm 0.16}t^{1.9\pm 0.4}$, while these features arise naturally from a misaligned relativistic jet. Such a bipolar, relativistic jet should have a large angular size ($\sim$ 2 mas) which could be resolved with VLBA, as well as a large degree of linear polarization ($\sim 10%-20%$), unlike radio SNe. The presence of a relativistic jet in SN 2001em would have interesting implications. It would suggest that several percent of SNe Ib/c produce mildly relativistic jets, with an initial Lorentz factor $\Gamma_0\gtrsim 2-3$, while the fraction that produce GRB jets (with $\Gamma_0\gtrsim 100$) is $\sim 100$ times smaller. This could considerably increase the expected number of orphan GRB afterglows in the radio, and to a lesser extent in the optical and X-rays, if there is a continuous distribution in $\Gamma_0$. Furthermore, this may give further credence to the idea that core collapse SNe, and in particular SNe Type Ib/c, are triggered by bipolar jets.