SN 2017gci: a nearby Type I Superluminous Supernova with a bumpy tail
Sollerman, J.; Gromadzki, M.; Jerkstrand, A.; Galbany, L.; Inserra, C.; Gutiérrez, C. P.; Howell, D. A.; Rau, A.; Chen, T. -W.; Kankare, E.; Young, D. R.; Cappellaro, E.; Benetti, S.; Lunnan, R.; Leloudas, G.; McCully, C.; Berton, M.; Fong, W.; Nicholl, M.; Burke, J.; Hiramatsu, D.; Valenti, S.; Hosseinzadeh, G.; Schulze, S.; Terreran, G.; Ciolfi, R.; Fiore, A.; Müller-Bravo, T. E.; Pastorello, A.; O'Neill, D.
Italy, Sweden, Germany, United Kingdom, Denmark, Israel, Finland, United States, Spain, Poland
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.