Old and Bright: The Remarkable Radio Brightening of the Engine-driven SN 2012au Several Years After Explosion Signals the Birth of a PWN

Abstract

We present the results from an extensive broad-band (radio to X-rays) observing campaign of the engine-driven Type Ib SN 2012au in the first 13 years of evolution. The early-time ($\hat{I}´t\le 190$ d) radio and X-ray evolution is well-described by conventional models of a forward shock interacting with a wind-like circumstellar medium ($\ddot{I}{}_{\mathrm{CSM}}\propto r^{-2}$). However, starting at $\hat{I}´t\approx 6.7$ yr, we detect a significant radio re-brightening. This late-time emission is dominated by a luminous component characterized by a broad and rapidly evolving spectral peak and a shallow optically thin spectral slope, $F_{\hat{I}}½\propto \hat{I}{½}^{-0.31\pm 0.02}$. These properties imply a compact emitting region ($R\lesssim {10}^{16}$ cm) expanding at a remarkably slow velocity ($\lesssim 500$ km/s) into a high-density environment ($\ge {10}^4{\mathrm{cm}}^{-3}$), accompanied by a hard electron power-law index $p\approx 1.6$. No soft or hard X-ray emission is detected at any epoch, indicating that high-energy radiation is either strongly absorbed or intrinsically absent. In the context of aspherical shock-CSM interaction models, these observations imply extreme properties of the CSM (geometry, density, total mass) that lack clear astrophysical motivation. Instead, we show that the emergence of radiation from a newborn Pulsar Wind Nebula (PWN) naturally explains the radio spectral evolution and high-energy limits, where the emission is governed by the adiabatic expansion of a relic pair plasma. We conclude that SN 2012au represents the most compelling candidate for a young, newborn PWN discovered to date, a scenario that can be directly tested with pending Very Long Baseline Interferometry (VLBI) observations.

39 pages, 20 figures, 4 tables, Submitted to ApJ