Padova-Asiago Supernova Group

Signatures of an eruptive phase before the explosion of the peculiar core-collapse SN 2013gc
Reguitti, A. et al. 2019, MNRAS 482, 2750
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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≈20. The early supernova spectra show Balmer lines, with a narrow (∼560 km s−1 ) P-Cygni absorption superimposed on a broad (∼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 Hα 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 massloss events, that we directly observed from 3 years before the explosio. The modest luminosity (MI ∼ −16.5 near maximum) of SN 2013gc at all phases, the very small amount of ejected 56Ni (of the order of 10−3 M ), 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. 187