SUPERLUMINOUS SUPERNOVA SN 2015bn IN THE NEBULAR PHASE: EVIDENCE FOR THE ENGINE-POWERED EXPLOSION OF A STRIPPED MASSIVE STAR

: Nicholl M, Berger E, Margutti R, Chornock R, Blanchard PK, Jerkstrand A, Smartt SJ, Arcavi I, Challis P, Chambers KC, Chen TW, Cowperthwaite PS, Gal-Yam A, Hosseinzadeh G, Howell DA, Inserra C, Kankare E, Magnier EA, Maguire K, Mazzali PA, McCully C, Milisavljevic D, Smith KW, Taubenberger S, Valenti S, Wainscoat RJ, Yaron O, Young DR

Publisher: IOP PUBLISHING LTD

: 2016

: Astrophysical Journal Letters

: ASTROPHYSICAL JOURNAL LETTERS

: ASTROPHYS J LETT

: ARTN L18

: 828

: 2

: 7

: 2041-8205

DOI: https://doi.org/10.3847/2041-8205/828/2/L18

We 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 Ni-56 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-circle dot of oxygen-dominated ejecta (for reasonable choices in temperature and opacity). A strong and relatively narrow O I lambda 7774 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.