The spectacular evolution of Supernova 1996al over 15 years: a low energy explosion of a stripped massive star in a highly structured environment

TL;DR

This 15-year study of Supernova 1996al reveals a low-energy explosion of a stripped massive star interacting with a highly asymmetric circumstellar environment, showing complex spectral evolution and fallback onto a black hole.

Contribution

The paper provides a detailed long-term spectrophotometric analysis of SN 1996al, highlighting its unique low-energy explosion and circumstellar interaction, which were not previously characterized in such detail.

Findings

SN 1996al is a Linear type-II supernova with peak magnitude ~ -18.2 mag.

The supernova ejecta was low mass (~1.15 Msun) with modest kinetic energy (~1.6 x 10^50 erg).

The progenitor was a massive star (~25 Msun ZAMS) that lost most of its hydrogen envelope.

Abstract

Spectrophotometry of SN 1996al carried out throughout 15 years is presented. The early photometry suggests that SN 1996al is a Linear type-II supernova, with an absolute peak of Mv ~ -18.2 mag. Early spectra present broad, asymmetric Balmer emissions, with super-imposed narrow lines with P-Cygni profile, and He I features with asymmetric, broad emission components. The analysis of the line profiles shows that the H and He broad components form in the same region of the ejecta. By day +142, the Halpha profile dramatically changes: the narrow P-Cygni profile disappears, and the Halpha is fitted by three emission components, that will be detected over the remaining 15 yrs of the SN monitoring campaign. Instead, the He I emissions become progressively narrower and symmetric. A sudden increase in flux of all He I lines is observed between 300 and 600 days. Models show that the supernova luminosity is sustained by the interaction of low mass (~1.15 Msun) ejecta, expelled in a low kinetic energy (~ 1.6 x 10^50 erg) explosion, with highly asymmetric circumstellar medium. The detection of Halpha emission in pre-explosion archive images suggests that the progenitor was most likely a massive star (~25 Msun ZAMS) that had lost a large fraction of its hydrogen envelope before explosion, and was hence embedded in a H-rich cocoon. The low-mass ejecta and modest kinetic energy of the explosion are explained with massive fallback of material into the compact remnant, a 7-8 Msun black hole.