VLBI Observations of SN 2012AU Reveal a Compact Radio Source a Decade Post Explosion

Three leading models have been put forth to justify the observed radio re-brightening associated with stripped-envelope supernovae (SESNe) years post-explosion: radiation from an emerging pulsar wind nebula (PWN), shock interaction with a dense circumstellar medium (CSM), or emission from off-axis, relativistic jets. SN 2012au is a particularly intriguing SESN in this regard, as observations obtained ≳6 yr post-explosion have shown both (i) optical emission features consistent with a young PWN and (ii) a radio re-brightening. We present the results of our very long baseline interferometric (VLBI) observations of SN 2012au performed between 8 and 13 yr post core-collapse. Our VLBI observations reveal a luminous, steadily fading radio source that remains compact (≤1.4 × 10¹⁷ cm) and stationary (≤0.36c) over the course of our campaign. Overall, we find that our VLBI measurements can be readily explained by a ∼decade-old PWN, potentially explained by shock interaction with specific CSM geometries, and are unlikely to be explained by emission from an off-axis, relativistic jet. Assuming a PWN origin, our observations require that the initial spindown luminosity of the central pulsar be between 10³⁶ erg s⁻¹ ≤ E₀ ̇ ≤ 4 × 10⁴² erg s⁻¹ and radio efficiency factor be η_R ≥ 3 × 10⁻⁷ (both quoted at the 99.7% confidence interval). These results are consistent with independent inferences obtained using optical spectroscopy of SN 2012au, alongside inferences of known Galactic systems. If a PWN origin is confirmed, SN 2012au would represent the first extragalactic PWN emerging from a modern-day SN, providing a novel opportunity to study the formation properties of a decade-old pulsar.