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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, 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. ORCID:, 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. and Young, D. R. 2016. Superluminous supernova SN 2015bn in the nebular phase: evidence for the engine-powered explosion of a stripped massive star. Astrophysical Journal Letters 828 (2) , L18. 10.3847/2041-8205/828/2/L18

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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.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Physics and Astronomy
Publisher: American Astronomical Society
ISSN: 2041-8205
Date of First Compliant Deposit: 23 January 2019
Date of Acceptance: 23 August 2016
Last Modified: 06 May 2023 02:10

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