Subaru and Keck Observations of the Peculiar Type Ia Supernova 2006gz at Late Phases

TL;DR

This study presents late-phase Subaru and Keck observations of the peculiar, overluminous Type Ia supernova 2006gz, revealing unique spectral features that challenge existing models of super-Chandrasekhar mass explosions.

Contribution

It provides the first late-time spectroscopic and photometric data of SN 2006gz, highlighting its distinct behavior and discussing possible explanations for its peculiar features.

Findings

SN 2006gz is fainter at late times than expected for a super-Chandrasekhar mass SN.

Lack of [Fe II] and [Fe III] emission suggests less radioactive nickel than typical for overluminous SNe.

Late-time spectral features may indicate emission shifted to longer wavelengths or different explosion mechanisms.

Abstract

Recently, a few peculiar Type Ia supernovae (SNe) that show exceptionally large peak luminosity have been discovered. Their luminosity requires more than 1 Msun of 56Ni ejected during the explosion, suggesting that they might have originated from super-Chandrasekhar mass white dwarfs. However, the nature of these objects is not yet well understood. In particular, no data have been taken at late phases, about one year after the explosion. We report on Subaru and Keck optical spectroscopic and photometric observations of the SN Ia 2006gz, which had been classified as being one of these "overluminous" SNe Ia. The late-time behavior is distinctly different from that of normal SNe Ia, reinforcing the argument that SN 2006gz belongs to a different subclass than normal SNe Ia. However, the peculiar features found at late times are not readily connected to a large amount of 56Ni; the SN is faint, and it lacks [Fe II] and [Fe III] emission. If the bulk of the radioactive energy escapes the SN ejecta as visual light, as is the case in normal SNe Ia, the mass of 56Ni does not exceed ~ 0.3 Msun. We discuss several possibilities to remedy the problem. With the limited observations, however, we are unable to conclusively identify which process is responsible. An interesting possibility is that the bulk of the emission might be shifted to longer wavelengths, unlike the case in other SNe Ia, which might be related to dense C-rich regions as indicated by the early-phase data. Alternatively, it might be the case that SN 2006gz, though peculiar, was actually not substantially overluminous at early times.