Early Spectral Evolution of the Rapidly Expanding Type Ia SN 2006X

TL;DR

This paper presents early spectral observations of SN 2006X, revealing extremely high expansion velocities and unusual spectral features that challenge existing temperature diagnostics for rapidly expanding Type Ia supernovae.

Contribution

It provides detailed spectroscopic data of SN 2006X at early phases and discusses the implications for supernova explosion models and temperature indicators.

Findings

SN 2006X has one of the highest expansion velocities among SNe Ia.

The low ratio of Si II lines suggests high temperature, but other spectral features imply low temperature.

The Si II 5972 line may be contaminated, questioning its use as a temperature indicator.

Abstract

We present optical spectroscopic and photometric observations of Type Ia supernova (SN) 2006X from --10 to +91 days after the $B$-band maximum. This SN exhibits one of the highest expansion velocity ever published for SNe Ia. At premaximum phases, the spectra show strong and broad features of intermediate-mass elements such as Si, S, Ca, and Mg, while the O{\sc i}$\lambda$7773 line is weak. The extremely high velocities of Si{\sc ii} and S{\sc ii} lines and the weak O{\sc i} line suggest that an intense nucleosynthesis might take place in the outer layers, favoring a delayed detonation model. Interestingly, Si{\sc ii}$\lambda$5972 feature is quite shallow, resulting in an unusually low depth ratio of Si{\sc ii}$\lambda$5972 to $\lambda$6355, $\cal R$(Si{\sc ii}). The low $\cal R$(Si{\sc ii}) is usually interpreted as a high photospheric temperature. However, the weak Si{\sc iii}$\lambda$4560 line suggests a low temperature, in contradiction to the low $\cal R$(Si{\sc ii}). This could imply that the Si{\sc ii}$\lambda$5972 line might be contaminated by underlying emission. We propose that $\cal R$(Si{\sc ii}) may not be a good temperature indicator for rapidly expanding SNe Ia at premaximum phases.