Superluminous Supernova SN 2015bn in the Nebular Phase: Evidence for the Engine-powered Explosion of a Stripped Massive Star

M. Nicholl, E. Berger, R. Margutti, R. Chornock, P. K. Blanchard, A. Jerkstrand, S. J. Smartt, I. Arcavi, P. Challis, K. C. Chambers, T. -W. Chen, P. S. Cowperthwaite, A. Gal-Yam, G. Hosseinzadeh, D. A. Howell, C. Inserra, E. Kankare, E. A. Magnier, K. Maguire, P. A. MazzaliC. McCully, D. Milisavljevic, K. W. Smith, S. Taubenberger, S. Valenti, R. J. Wainscoat, O. Yaron, D. R. Young

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Abstract

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 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....
Original languageUndefined/Unknown
JournalAstrophysical Journal Letters
DOIs
Publication statusPublished - Sep 2016

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