Radio Analysis of SN2004C Reveals an Unusual CSM Density Profile as a Harbinger of Core Collapse

We present extensive multifrequency Karl G. Jansky Very Large Array (VLA) and Very Long Baseline Array (VLBA) observations of the radio-bright supernova (SN) IIb SN 2004C that span similar to 40-2793 days post-explosion. We interpret the temporal evolution of the radio spectral energy distribution in the context of synchrotron self-absorbed emission from the explosion's forward shock as it expands in the circumstellar medium (CSM) previously sculpted by the mass-loss history of the stellar progenitor. VLBA observations and modeling of the VLA data point to a blastwave with average velocity similar to 0.06 c that carries an energy of approximate to 10(49) erg. Our modeling further reveals a flat CSM density profile rho (CSM) proportional to R-0.03 +/- 0.22 up to a break radius R (br) approximate to (1.96 +/- 0.10) x 10(16) cm, with a steep density gradient following rho (CSM) proportional to R-2.3 +/- 0.5 at larger radii. We infer that the flat part of the density profile corresponds to a CSM shell with mass similar to 0.021 M-circle dot, and that the progenitor's effective mass-loss rate varied with time over the range (50-500) x 10(-5) M(circle dot)yr(-1) for an adopted wind velocity v(w) = 1000 km s(-1) and shock microphysical parameters epsilon(e) = 0.1, epsilon(B) = 0.01. These results add to the mounting observational evidence for departures from the traditional single-wind mass-loss scenarios in evolved, massive stars in the centuries leading up to core collapse. Potentially viable scenarios include mass loss powered by gravity waves and/or interaction with a binary companion.