Superluminous supernova 2015bn in the nebular phase: evidence for the engine-powered explosion of a stripped massive star

TL;DR

This study presents nebular-phase observations of superluminous supernova SN 2015bn, providing evidence for an engine-powered explosion of a stripped massive star, linking it to hypernovae and gamma-ray bursts.

Contribution

It offers the first detailed nebular spectra and light curve analysis of SN 2015bn, supporting a unified engine-driven explosion model for superluminous supernovae.

Findings

Spectral evolution from photospheric to nebular phase with emission lines of oxygen, calcium, magnesium.

Spectra similar to energetic Type Ic supernovae, linking superluminous supernovae to hypernovae.

Evidence for an engine-driven explosion ejecting 7-30 solar masses of oxygen-dominated material.

Abstract

We present nebular-phase imaging and spectroscopy for the hydrogen-poor superluminous supernova SN 2015bn, at redshift z=0.1136, spanning +250-400 d after maximum light. The light curve exhibits a steepening in the decline rate from 1.4 mag/(100 d) to 1.7 mag/(100 d), 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 nickel mass. We show that the spectrum at +400 d is virtually identical to a number of energetic Type Ic supernovae such as SN 1997dq, SN 2012au, and SN 1998bw, indicating similar core conditions and strengthening the link between `hypernovae'/long gamma-ray bursts and superluminous supernovae. 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$_\odot$ of oxygen-dominated ejecta (for reasonable choices in temperature and opacity). A strong and relatively narrow O II $\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.