Nicholl, MatthewBerger, EdoMargutti, R.Chornock, R.Blanchard, P. K.Jerkstrand, A.Smartt, S. J.Arcavi, I.Challis, P.Chambers, K. C.Chen, T.-W.Cowperthwaite, P. S.Gal-Yam, A.Hosseinzadeh, G.Howell, D. A.Inserra, C.Kankare, E.Magnier, E. A.Maguire, K.Mazzali, P. A.McCully, C.Milisavljevic, D.Smith, K. W.Taubenberger, S.Valenti, S.Wainscoat, R. J.Yaron, O.Young, D. R.2017-03-062016Nicholl, M., E. Berger, R. Margutti, R. Chornock, P. K. Blanchard, A. Jerkstrand, S. J. Smartt, et al. 2016. Superluminous Supernova Sn 2015bn in the Nebular Phase: Evidence for the Engine-Powered Explosion of a Stripped Massive Star. The Astrophysical Journal 828, no. 2: L18. doi:10.3847/2041-8205/828/2/l18.2041-8213http://nrs.harvard.edu/urn-3:HUL.InstRepos:30510302We present nebular-phase imaging and spectroscopy for the hydrogen-poor superluminous supernova (SLSN) SN 2015bn, at redshift z = 0.1136, spanning +250–400 days after maximum light. The light curve exhibits a steepening in the decline rate from 1.4 mag (100 days)−1 to 1.7 mag (100 days)−1, suggestive of a significant decrease in the opacity. This change is accompanied by a transition from a blue continuum superposed with photospheric absorption lines to a nebular spectrum dominated by emission lines of oxygen, calcium, and magnesium. There are no obvious signatures of circumstellar interaction or large 56Ni mass. We show that the spectrum at +400 days is virtually identical to a number of energetic SNe Ic such as SN 1997dq, SN 2012au, and SN 1998bw, indicating similar core conditions and strengthening the link between "hypernovae"/long gamma-ray bursts and SLSNe. A single explosion mechanism may unify these events that span absolute magnitudes of −22 < M B < −17. Both the light curve and spectrum of SN 2015bn are consistent with an engine-driven explosion ejecting 7–30 M ⊙ of oxygen-dominated ejecta (for reasonable choices in temperature and opacity). A strong and relatively narrow O i λ7774 line, seen in a number of these energetic events but not in normal supernovae, may point to an inner shell that is the signature of a central engine.en-USsupernovae: generalsupernovae: 2015bnJournal Article2017-03-0610.3847/2041-8205/828/2/l18