SN 2003bg: The First Type IIb Hypernova

TL;DR

SN 2003bg is identified as the first Type IIb hypernova, exhibiting features of both Type IIb and hypernovae, with broad light curves and high kinetic energy indicating a massive progenitor.

Contribution

This paper presents the first detailed observations and analysis of SN 2003bg as a Type IIb hypernova, combining photometry and spectroscopy over 300 days.

Findings

Broad, high-velocity Balmer lines in early spectra

Nebular spectra resemble Type Ib/c supernovae without hydrogen

Broad bolometric light curve suggests a large progenitor mass

Abstract

Optical and near-infrared photometry and optical spectroscopy are reported for SN 2003bg, starting a few days after explosion and extending for a period of more than 300 days. Our early-time spectra reveal the presence of broad, high-velocity Balmer lines. The nebular-phase spectra, on the other hand, show a remarkable resemblance to those of Type Ib/c supernovae, without clear evidence for hydrogen. Near maximum brightness SN 2003bg displayed a bolometric luminosity comparable to that of other Type I hypernovae unrelated to gamma-ray bursts, implying a rather normal amount of 56Ni production (0.1-0.2 Msun) compared with other such objects. The bolometric light curve of SN 2003bg, on the other hand, is remarkably broad, thus suggesting a relatively large progenitor mass at the moment of explosion. These observations, together with the large value of the kinetic energy of expansion established in the accompanying paper (Mazzali et al. 2009), suggest that SN 2003bg can be regarded as a Type IIb hypernova.