We present extensive radio observations of the nearby Type Ibc supernovae ( SNe Ibc ) 2004cc , 2004dk , and 2004gq spanning \Delta t \approx 8 - 1900 days after explosion .
Using a dynamical model developed for synchrotron emission from a slightly decelerated shockwave , we estimate the velocity and energy of the fastest ejecta and the density profile of the circumstellar medium .
The shockwaves of all three supernovae are characterized by non-relativistic velocities of \overline { v } \approx ( 0.1 - 25 ) c and associated energies of E \approx ( 2 - 10 ) \times 10 ^ { 47 } erg , in line with the expectations for a typical homologous explosion .
Smooth circumstellar density profiles are indicated by the early radio data and we estimate the progenitor mass loss rates to be \dot { M } \approx ( 0.6 - 13 ) \times 10 ^ { -5 } ~ { } M _ { \odot } ~ { } yr ^ { -1 } ( wind velocity , v _ { w } = 10 ^ { 3 } ~ { } km~ { } s ^ { -1 } ) .
These estimates approach the saturation limit ( \dot { M } \approx 10 ^ { -4 } ~ { } M _ { \odot } ~ { } yr ^ { -1 } ) for line-driven winds from Wolf-Rayet stars , the favored progenitors of SNe Ibc including those associated with long-duration gamma-ray bursts .
Intriguingly , at later epochs all three supernovae show evidence for abrupt radio variability that we attribute to large density modulations ( factor of \sim 3 - 6 ) at circumstellar radii of r \approx ( 1 - 50 ) \times 10 ^ { 16 } cm .
If due to variable mass loss , these modulations are associated with progenitor activity on a timescale of \sim 10 - 100 years before explosion .
We consider these results in the context of variable mass loss mechanisms including wind clumping , metallicity-independent continuum-driven ejections , and binary-induced modulations .
It may also be possible that the SN shockwaves are dynamically interacting with wind termination shocks , however , this requires the environment to be highly pressurized and/or the progenitor to be rapidly rotating prior to explosion .
The proximity of the density modulations to the explosion sites may suggest a synchronization between unusual progenitor mass loss and the SN explosion , reminiscent of Type IIn supernovae .
This study underscores the utility of radio observations for tracing the final evolutionary stage ( s ) of SN progenitor systems .