Supernova Interaction with a Dense Detached Shell in SN 2001em

TL;DR

This comprehensive 19-year study of supernova SN 2001em reveals its transformation from a Type Ib supernova to an interacting supernova with a dense circumstellar shell, highlighting the role of binary evolution in its progenitor's mass loss.

Contribution

First detailed long-term multi-wavelength analysis of SN 2001em showing its metamorphosis and interaction with a dense shell, suggesting binary progenitor evolution.

Findings

Shock exited the dense shell around 1750 days.

X-ray emission likely from forward shock.

Late-time radio spectral inversion indicates possible binary progenitor.

Abstract

We carry out a comprehensive analysis of supernova \snem\ covering a period of 19 years since discovery. SN 2001em is the oldest supernova known to have undergone a metamorphosis from a stripped envelope to an interacting supernova. An early spectrum indicates it exploded as a Type Ib supernova. Later, the ejecta caught up with a dense circumstellar H-shell, ejected a few thousand years before the explosion, triggering interaction between the supernova ejecta and the dense shell, producing radio, X-ray and H$\alpha$ emission. We use archival data with the Very Large Array in radio bands and with Chandra, XMM-NEWTON and SWIFT-XRT in X-ray bands, along with published H$\alpha$ measurements. We combine these data with our low radio frequency observations with the Giant Metrewave Radio Telescope at two epochs covering three frequencies. While the observations missed the phase when the shock entered the dense shell, the X-rays indicate that the shock came out of the dense shell at around 1750 days. The data suggest a forward shock origin of the X-ray emission. Radio data show a spectral inversion at late epochs ($> 5000$\,d) at around 3 GHz, which mimics the properties of the central absorbed component seen in SN 1986J. A possible explanation for this component is that the progenitor of \snem\ was a massive binary system which had undergone a period of common envelope evolution. The hydrogen envelope from the \snem\ progenitor may have been lost as a result of binary interaction.