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We present extensive multi-frequency VLA and VLBA observations of theradio-bright supernova (SN) IIb SN 2004C that span $\sim(40-2793)$ dayspost-explosion. We interpret the temporal evolution of the radio spectralenergy distribution (SED) in the context of synchrotron self-absorbed (SSA)emission from the explosion's forward shock as it expands in the circumstellarmedium (CSM) previously sculpted by the mass-loss history of the stellarprogenitor. VLBA observations and modeling of the VLA data point to a blastwavewith average velocity $\sim0.06c$ that carries an energy of $\sim 10^{49}$ erg.Our modeling further reveals a flat CSM density profile $\rho_{\rm{CSM}}\propto R^{-0.03 \pm0.22}$ up to a break radius $R_{br} \approx (1.96 \pm 0.10)\times 10^{16}$ cm, with a steep density gradient following $\rho_{\rm{CSM}}\propto R^{-2.3 \pm 0.5}$ at larger radii. We infer that the flat part of thedensity profile corresponds to a CSM shell with mass $\sim0.021 M_{\odot}$, andthat the progenitor's effective mass-loss rate varied with time over the range$(50-500) \times 10^{-5} M_{\odot} \rm{yr}^{-1}$ 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 evidencefor departures from the traditional single-wind mass-loss scenarios in evolved,massive stars in the centuries leading up to core collapse. Potentially viablescenarios include mass loss powered by gravity waves and/or interaction with abinary companion.

Radio Analysis of SN 2004C Reveals an Unusual CSM Density Profile as a Harbinger of Core Collapse