2016ApJ...826...39N

We present observations of SN 2015bn (=PS15ae=CSS141223-113342+004332=MLS150211-113342+004333), a Type I superluminous supernova (SLSN) at redshift z=0.1136. As well as being one of the closest SLSNe I yet discovered, it is intrinsically brighter ( {M}_{U}~-23.1 ) and in a fainter galaxy ( {M}_{B}~-16.0 ) than other SLSNe at z∼0.1 . We used this opportunity to collect the most extensive data set for any SLSN I to date, including densely sampled spectroscopy and photometry, from the UV to the NIR, spanning -50 to +250 days from optical maximum. SN 2015bn fades slowly, but exhibits surprising undulations in the light curve on a timescale of 30-50 days, especially in the UV. The spectrum shows extraordinarily slow evolution except for a rapid transformation between +7 and +20-30 days. No narrow emission lines from slow-moving material are observed at any phase. We derive physical properties including the bolometric luminosity, and find slow velocity evolution and non-monotonic temperature and radial evolution. A deep radio limit rules out a healthy off-axis gamma-ray burst, and places constraints on the pre-explosion mass loss. The data can be consistently explained by a \gtrsim 10 M _☉ stripped progenitor exploding with ∼{10}^{51} ergkinetic energy, forming a magnetar with a spin-down timescale of ∼20 days (thus avoiding a gamma-ray burst) that reheats the ejecta and drives ionization fronts. The most likely alternative scenario-interaction with ∼20 M _☉ of dense, inhomogeneous circumstellar material-can be tested with continuing radio follow-up.