Demystifying shock breakout spectra

TL;DR

This paper presents a simplified model for classifying shock breakout spectra in supernovae, linking spectral features to physical time-scales and enabling better interpretation of early supernova observations.

Contribution

It introduces a framework that categorizes early supernova spectra based on the ordering of key physical time-scales, providing a basis for interpreting shock breakout observations.

Findings

Five spectral scenarios identified based on time-scale orderings.

Spectral evolution described by broken power-laws with specific indices.

Model enables constraining progenitor conditions from early observations.

Abstract

The spectrum of the first supernova light (i.e., the shock breakout and early cooling emission) is an important diagnostic for the state of the progenitor star just before explosion. We consider a streamlined model describing the emergent shock breakout spectrum, which enables a straightforward classification of the possible observed spectra during the early planar phase. The overall spectral evolution is determined by a competition between three important time-scales: the diffusion time $t_{\rm{bo}}$ of the shell producing the breakout emission, the light-crossing time of this shell $t_{\rm{lc}}$, and the time $t_{\rm{eq}}$ at which the observer starts to see layers of the ejecta where the gas and radiation are in thermal equilibrium. There are five allowed orderings of these time-scales, resulting in five possible scenarios with distinct spectral behaviours. Within each scenario, the spectrum at a given time is one of five possible types, which are approximately described by broken power-laws; we provide the spectral and temporal indices of each power-law segment, and the time evolution of the break frequencies. If high-cadence multi-wavelength observations can determine the relevant breakout scenario in future events, strong constraints can be placed on the physical conditions at the site of shock breakout.