We present extensive radio observations of SN 2003L , the most luminous and energetic Type Ibc radio supernova with the exception of SN 1998bw ( associated with GRB 980425 ) .
Observations from the Very Large Array are well described by a fitting a synchrotron self-absorption model to the emission spectrum .
This model implies a sub-relativistic ejecta velocity , \overline { v } \approx 0.2 c , and a size of r \approx 4.3 \times 10 ^ { 15 } cm at t \approx 10 days .
The circumstellar density is suitably fit with a stellar wind profile , n _ { e } \propto r ^ { -2 } and a constant mass loss rate of \dot { M } \approx 7.5 \times 10 ^ { -6 } ~ { } M _ { \odot } ~ { } yr ^ { -1 } .
Moreover , the magnetic field follows B \propto r ^ { -1 } and the kinetic energy of the radio bright ejecta is roughly E \approx 10 ^ { 48 } erg assuming equipartition of energy between relativistic electrons and magnetic fields .
Furthermore , we show that free-free absorption does not contribute significantly to the radio spectrum , since it implies ejecta velocities which are inconsistent with size constraints derived from Very Long Baseline Array observations .
In conclusion , we find that although SN 2003L has a radio luminosity comparable to that seen in SN 1998bw , it shows no evidence for a significant amount of energy coupled to relativistic ejecta .
Using SN 2003L as an example , we comment briefly on the coupling of ejecta velocity and energy in Type Ibc supernovae .