SN 2015bn: A DETAILED MULTI-WAVELENGTH VIEW OF A NEARBY SUPERLUMINOUS SUPERNOVA

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[subscript U] ≈ -23.1) and in a fainter galaxy (M[subscript 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 ≳ 10 M [subscript ⊙] stripped progenitor exploding with ∼10 [superscript 51] erg kinetic 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 [subscript ⊙] of dense, inhomogeneous circumstellar material—can be tested with continuing radio follow-up.United States. National Aeronautics and Space Administration. (Grant No. NNX08AR22G)National Science Foundation (U.S.) (Grant No. AST-1238877)United States. National Aeronautics and Space Administration. (Grant No. NNX12AR65G)United States. National Aeronautics and Space Administration. (Grant No. NNX14AM74G)Ernest Rutherford FellowshipUnited States. National Aeronautics and Space Administration. (Hubble Fellowship grant HF-51348.001)United States. Department of Energy. (grant number DE-FG02-97ER25308)National Science Foundation (U.S.) (grant AST-1410950)National Science Foundation (U.S.) (grant AST-0908816)National Science Foundation (U.S.) (AST-1515927)National Science Foundation (U.S.) (grant AST-1515876)National Science Foundation (U.S.) (grant DGE1144152)National Science Foundation (U.S.) (grant 1313484