Signatures of an eruptive phase before the explosion of the peculiar core-collapse SN 2013gc

TL;DR

SN 2013gc exhibited signs of pre-explosion activity and complex circumstellar material, with spectral and photometric features indicating a fall-back core-collapse origin and a peculiar type IId supernova classification.

Contribution

This study provides the first detailed analysis of SN 2013gc's pre-explosion activity, spectral features, and circumstellar environment, classifying it as a type IId supernova with a fall-back core-collapse origin.

Findings

Pre-explosion activity observed 1.5-2.5 years before explosion.

Complex circumstellar medium with spherically symmetric and bilobed components.

Low ejected $^{56}$Ni mass (~10^{-3} M$_\odot$) indicating fall-back core-collapse.

Abstract

We present photometric and spectroscopic analysis of the peculiar core-collapse SN 2013gc, spanning seven years of observations. The light curve shows an early maximum followed by a fast decline and a phase of almost constant luminosity. At +200 days from maximum, a brightening of 1 mag is observed in all bands, followed by a steep linear luminosity decline after +300 d. In archival images taken between 1.5 and 2.5 years before the explosion, a weak source is visible at the supernova location, with mag$\approx$20. The early supernova spectra show Balmer lines, with a narrow ($\sim$560 km s$^{-1}$) P-Cygni absorption superimposed on a broad ($\sim$3400 km s$^{-1}$) component, typical of type IIn events. Through a comparison of colour curves, absolute light curves and spectra of SN 2013gc with a sample of supernovae IIn, we conclude that SN 2013gc is a member of the so-called type IId subgroup. The complex profile of the H$\alpha$ line suggests a composite circumstellar medium geometry, with a combination of lower velocity, spherically symmetric gas and a more rapidly expanding bilobed feature. This circumstellar medium distribution has been likely formed through major mass-loss events, that we directly observed from 3 years before the explosion. The modest luminosity ($M_I\sim-16.5$ near maximum) of SN 2013gc at all phases, the very small amount of ejected $^{56}$Ni (of the order of $10^{-3}$ M$_\odot$), the major pre-supernova stellar activity and the lack of prominent [O I] lines in late-time spectra support a fall-back core-collapse scenario for the massive progenitor of SN~2013gc.