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

Abstract

We present extensive multi-frequency VLA and VLBA observations of the radio-bright supernova (SN) IIb SN 2004C that span $\sim (40-2793)$ days post-explosion. We interpret the temporal evolution of the radio spectral energy distribution (SED) in the context of synchrotron self-absorbed (SSA) 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 $\sim 0.06c$ that carries an energy of $\sim 10^{49}$ erg. Our modeling further reveals a flat CSM density profile $\ddot{I}{}_{\mathrm{CSM}}\propto R^{-0.03\pm 0.22}$ up to a break radius $R_{br}\approx (1.96\pm 0.10)\times 10^{16}$ cm, with a steep density gradient following $\ddot{I}{}_{\mathrm{CSM}}\propto R^{-2.3\pm 0.5}$ at larger radii. We infer that the flat part of the density profile corresponds to a CSM shell with mass $\sim 0.021M_{\odot }$, and that the progenitor's effective mass-loss rate varied with time over the range $(50-500)\times 10^{-5}{M}_{\odot }{\mathrm{yr}}^{-1}$ for an adopted wind velocity $v_w=1000$ km $s^{-1}$ and shock microphysical parameters $\hat{I}{\mu }_e=0.1,\hat{I}{\mu }_B=0.01$. 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.

32 pages, 10 figures, submitted to ApJ