# SN 2017gci: a nearby Type I Superluminous Supernova with a bumpy tail

A Fiore, T-W Chen, A Jerkstrand, S Benetti, R Ciolfi, C Inserra, E Cappellaro, A Pastorello, G Leloudas, S Schulze, M Berton, J Burke, C McCully, W Fong, L Galbany, M Gromadzki, C P Gutiérrez, D Hiramatsu, G Hosseinzadeh, D A HowellE Kankare, R Lunnan, T E Müller-Bravo, D O’ Neill, M Nicholl, A Rau, J Sollerman, G Terreran, S Valenti, D R Young

Research output: Contribution to journalArticlepeer-review

## Abstract

We present and discuss the optical spectrophotometric observations of the nearby (z = 0.087) Type I superluminous supernova (SLSN I) SN 2017gci, whose peak K-corrected absolute magnitude reaches Mg = −21.5 mag. Its photometric and spectroscopic evolution includes features of both slow- and of fast-evolving SLSN I, thus favoring a continuum distribution between the two SLSN-I subclasses. In particular, similarly to other SLSNe I, the multiband light curves (LCs) of SN 2017gci show two re-brightenings at about 103 and 142 d after the maximum light. Interestingly, this broadly agrees with a broad emission feature emerging around 6520 Å after ∼51 d from the maximum light, which is followed by a sharp knee in the LC. If we interpret this feature as Hα, this could support the fact that the bumps are the signature of late interactions of the ejecta with a (hydrogen-rich) circumstellar material. Then we fitted magnetar- and CSM-interaction-powered synthetic LCs on to the bolometric one of SN 2017gci. In the magnetar case, the fit suggests a polar magnetic field Bp ≃ 6 × 1014 G, an initial period of the magnetar Pinitial ≃ 2.8 ms, an ejecta mass $M_{\rm ejecta}\simeq 9\, \mathrm{M}_\odot$ and an ejecta opacity $\kappa \simeq 0.08\, \mathrm{cm}^{2}\, \rm{g}^{-1}$. A CSM-interaction scenario would imply a CSM mass $\simeq 5\, \mathrm{M}_\odot$ and an ejecta mass $\simeq 12\, \mathrm{M}_\odot$. Finally, the nebular spectrum of phase + 187 d was modeled, deriving a mass of $\sim 10\, {\rm M}_\odot$ for the ejecta. Our models suggest that either a magnetar or CSM interaction might be the power sources for SN 2017gci and that its progenitor was a massive ($40\, {\rm M}_\odot$) star.
Original language English 2120-2139 20 Monthly Notices of the Royal Astronomical Society 502 2 05 Jan 2021 https://doi.org/10.1093/mnras/staa4035 Published - Apr 2021

## Keywords

• Space and Planetary Science
• Astronomy and Astrophysics

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