SN 2017gci: a nearby type I superluminous supernova with a bumpy tail

We present and discuss the optical spectro-photometric 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 multi-band light curves of SN 2017gci show two re-brightenings at about 103 and 142 days after the maximum light. Interestingly, this broadly agrees with a broad emission feature emerging around 6520 Å after ∼51 days from the maximum light, which is followed by a sharp knee in the light curve. 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 light curves onto 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 Mejecta ≃ 9 M⊙ and an ejecta opacity κ ≃ 0.08 cm2 g−1. A CSM-interaction scenario would imply a CSM mass ≃ 5 M⊙ and an ejecta mass ≃ 12 M⊙. Finally, the nebular spectrum of phase +187 days was modeled, deriving a mass of ∼10 M⊙ 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 M⊙) star