A 14-year-old Mystery: The Peculiar Case of the Engine-driven SN 2012ap

TL;DR

This study investigates the late-time optical, X-ray, and radio observations of SN 2012ap, exploring whether its radio rebrightening is due to a density enhancement or an off-axis relativistic jet, challenging previous notions of it being a weak engine-driven explosion.

Contribution

The paper provides a comprehensive multi-wavelength analysis of SN 2012ap, proposing that its late-time radio rebrightening could indicate an off-axis jet, thus reclassifying its explosion nature.

Findings

Radio rebrightening could be due to density enhancement or off-axis jet.

Late-time optical spectra lack broad emission lines typical of interacting SNe.

An off-axis jet scenario suggests SN 2012ap is similar to GRBs.

Abstract

We present late-time ($\delta t > 3000$ d) optical (Keck), X-ray (Chandra and NuSTAR), and radio (VLA, ALMA, and the uGMRT) observations of the Type Ic-BL SN 2012ap. Previous studies of this SN suggested that it stands out as a key example of a weak engine-driven explosion due to the lack of gamma-ray burst detection and a mildly relativistic ejecta. Recently, radio sky surveys revealed the rebrightening of the radio emission from this SN, highlighting the possibilities of a density enhancement at large radii or the existence of an off-axis relativistic jet. While the late-time optical spectra does not exhibit the broad emission lines seen in other interacting SNe, our analysis of the broadband radio and X-ray emission implies that both scenarios are plausible. If a density enhancement is responsible for the radio rebrightening, it has to result from a change in the mass-loss rate and/or wind velocity, possibly due to the transition of the progenitor from a red supergiant to a Wolf-Rayet star. If the late-time radio component is a result of an off-axis relativistic jet, we find that an energetic narrow jet viewed at $\theta_{\rm obs} \geq 80^{\circ}$ is needed. In this scenario, SN 2012ap is not a result of a weak engine-driven explosion, and, instead, it is similar to other GRBs. However, radio rebrightenings of Type Ic-BL SNe are not enough on their own to determine the existence of off-axis jets and our planned VLBA observation will help reveal the true nature of this SN.