2022ApJ...938...84D

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 ∼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 ∼0.06 c that carries an energy of ≈10⁴⁹ erg. Our modeling further reveals a flat CSM density profile ρ_CSM ∝ R^{–0.03±0.22} up to a break radius R_br ≈ (1.96 ± 0.10) × 10¹⁶ cm, with a steep density gradient following ρ_CSM ∝ 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 ∼0.021 M_☉, and that the progenitor's effective mass-loss rate varied with time over the range (50-500) × 10^–5M_☉ yr^–1 for an adopted wind velocity v_w = 1000 km s^–1 and shock microphysical parameters ϵ_e = 0.1, ϵ_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.