Old and Bright: The Remarkable Radio Brightening of the Engine-driven SN 2012au Several Years after Explosion Signals the Birth of a Pulsar Wind Nebula

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Abstract We present the results from an extensive broadband (radio to X-rays) observing campaign of the engine-driven Type Ib SN 2012au in the first 13 yr of evolution. The early time ( δt ≤ 190 days) radio and X-ray evolution is well described by conventional models of a forward shock interacting with a wind-like circumstellar medium (CSM; ρ CSM ∝ r −2 ). However, starting at δt ≈ 6.7 yr, we detect a significant radio rebrightening. 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 ν ∝ ν −0.31 ± 0.02 . These properties imply a compact emitting region ( R ≲ 10 16 cm) expanding at a remarkably slow velocity ( v s ≲ 500 km s −1 ) into a high-density environment ( n e ≥ 10 4 cm −3 ), accompanied by a hard electron power-law index p ≈ 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, and 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 observations.

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