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

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 M_g = -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 B_p ≃ 6 × 10¹⁴ G, an initial period of the magnetar P_initial ≃ 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.