Supernova Interaction with a Dense Detached Shell in SN 2001em

Abstract

We carry out a comprehensive analysis of supernova SN 2001em 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 Hα 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 Hα 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 SN 2001em was a massive binary system which had undergone a period of common envelope evolution.

Two stripped envelope supernovae with circumstellar interaction -- but only one really shows it

“…Finally, we must mention SN 2014C (e.g., Milisavljevic et al 2015) which is a well studied SE SN that ran into CSM and which transformed from a Type Ib to a Type IIn SN, just like SN 2019oys, a phenomenon also seen in SN 2001em (Chugai & Chevalier 2006;Chandra et al 2020). Also in this case, late-time high-resolution spectra revealed coronal lines.…”

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“…SN 2001em, initially classified as type Ib SN, presented prominent Hα emission lines at 2.5 yrs. Associated with strong radio and X-ray emission, this was interpreted as evidence of the interaction between the SN ejecta and a massive (∼ 3 M ) hydrogen-rich shell located at ∼ 7 × 10 16 cm (Chugai & Chevalier 2006;Chandra et al 2020). Several other SNe show similar signs of early interaction with massive shells (see, e.g., Anupama et al 2005;Moriya & Maeda 2014;Chen et al 2018;Pooley et al 2019;Dwarkadas et al 2010;Ben-Ami et al 2014;Mauerhan et al 2018;Suzuki et al 2021).…”