SN 2001em , identified as a Type Ic supernova , has recently been detected in the radio and X-rays , \gtrsim 2 yr after the explosion .
The high luminosities at such late times 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 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 radio 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 .
The high X-ray luminosity provides an independent and more robust constraint – it requires \sim 10 ^ { 51 } erg in mildly relativistic ejecta .
The source should therefore currently have a large angular size ( \sim 2 mas ) which could be resolved in the radio with VLBA .
It is also expected to be bipolar and is thus likely to exhibit a large degree of linear polarization ( \sim 10 \% - 20 \% ) .
The presence of a relativistic outflow 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 , 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 transients similar to 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 .