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

TL;DR

The study investigates the origin of the late-time radio re-brightening in SN 2012au, testing whether it arises from an extrinsic young pulsar wind nebula (PWN), delayed shock interaction with circumstellar material (CSM), or emission from an off-axis relativistic jet. A multi-epoch VLBI campaign using the EVN and VLBA at $5$, $22$, and $24$ GHz from 2020–2025 achieves mas-scale imaging at 8–13 years post-explosion, enabling direct constraints on size and motion. The VLBI results indicate a luminous, compact source unresolved on VLBI scales, stationary within uncertainties (proper motion $\u2264 0.36c$) and gradually fading, favoring a decade-old PWN origin and challenging the off-axis jet scenario. If the PWN interpretation holds, the data require a central pulsar initial spin-down luminosity between $10^{36}$ erg s$^{-1}$ and $10^{42}$ erg s$^{-1}$ and radio efficiency $_R\ge 3\times 10^{-6}$, consistent with optical inferences and Galactic analogs, making SN 2012au a potential first extragalactic PWN emerging from a modern-day SN.

Abstract

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 $\gtrsim$ 6 years post-explosion have shown \emph{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 to 13 years post core-collapse. Our VLBI observations reveal a luminous, steadily fading radio source that remains compact ($\leq1.4\times10^{17}~\mathrm{cm}$) and stationary ($\leq0.36c$) over the course of our campaign. Overall, we find that our VLBI measurements can be readily explained by a $\sim$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 spin-down luminosity of the central pulsar be between $10^{36}~\mathrm{erg~s^{-1}}\leq\dot{E}_0\leq10^{42}~\mathrm{erg~s^{-1}}$ and radio efficiency factor be $η_\mathrm{R}\geq3\times10^{-6}$. 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.