%AFiore, A%AChen, T-W%AJerkstrand, A%ABenetti, S%ACiolfi, R%AInserra, C%ACappellaro, E%APastorello, A%ALeloudas, G%ASchulze, S%ABerton, M%ABurke, J%AMcCully, C%AFong, W%AGalbany, L%AGromadzki, M%AGutiérrez, C%AHiramatsu, D%AHosseinzadeh, G%AHowell, D%AKankare, E%ALunnan, R%AMüller-Bravo, T%AO’ Neill, D%ANicholl, M%ARau, A%ASollerman, J%ATerreran, G%AValenti, S%AYoung, D%Anull Ed.%BJournal Name: Monthly Notices of the Royal Astronomical Society; Journal Volume: 502; Journal Issue: 2 %D2021%I %JJournal Name: Monthly Notices of the Royal Astronomical Society; Journal Volume: 502; Journal Issue: 2 %K %MOSTI ID: 10290298 %PMedium: X %TSN 2017gci: a nearby Type I Superluminous Supernova with a bumpy tail %XABSTRACT 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. %0Journal Article