SN Refsdal: Classification as a Luminous and Blue SN 1987A-like Type II Supernova

TL;DR

SN Refsdal is classified as a luminous, blue, SN 1987A-like Type II supernova based on multi-epoch HST and VLT spectra and images, showing characteristic light curves, spectral features, and velocities at redshift 1.49.

Contribution

This study provides the first detailed spectral and photometric analysis of SN Refsdal, confirming its classification as a SN 1987A-like supernova at high redshift.

Findings

Light curve matches SN 1987A-like supernovae.

Spectral features show broad H-alpha P-Cygni profile.

Estimated ejecta mass of 20±5 solar masses.

Abstract

We have acquired Hubble Space Telescope (HST) and Very Large Telescope near-infrared spectra and images of supernova (SN) Refsdal after its discovery as an Einstein cross in Fall 2014. The HST light curve of SN Refsdal matches the distinctive, slowly rising light curves of SN 1987A-like supernovae (SNe), and we find strong evidence for a broad H-alpha P-Cygni profile in the HST grism spectrum at the redshift (z = 1.49) of the spiral host galaxy. SNe IIn, powered by circumstellar interaction, could provide a good match to the light curve of SN Refsdal, but the spectrum of a SN IIn would not show broad and strong H-alpha absorption. From the grism spectrum, we measure an H-alpha expansion velocity consistent with those of SN 1987A-like SNe at a similar phase. The luminosity, evolution, and Gaussian profile of the H-alpha emission of the WFC3 and X-shooter spectra, separated by ~2.5 months in the rest frame, provide additional evidence that supports the SN 1987A-like classification. In comparison with other examples of SN 1987A-like SNe, SN Refsdal has a blue B-V color and a high luminosity for the assumed range of potential magnifications. If SN Refsdal can be modeled as a scaled version of SN 1987A, we estimate it would have an ejecta mass of 20+-5 solar masses. The evolution of the light curve at late times will provide additional evidence about the potential existence of any substantial circumstellar material (CSM). Using MOSFIRE and X-shooter spectra, we estimate a subsolar host-galaxy metallicity (8.3+-0.1 dex and <8.4 dex, respectively) near the explosion site.