Monte-Carlo methods for NLTE spectral synthesis of supernovae

TL;DR

This paper introduces JEKYLL, a Monte-Carlo based code for detailed time-dependent NLTE spectral synthesis of supernovae, capable of modeling spectra and lightcurves with non-thermal processes, and demonstrates its accuracy and importance through comparisons and a Type IIb SN example.

Contribution

The paper extends the Monte-Carlo radiative transfer method to include full time-dependent NLTE effects, non-thermal processes, and material mixing, providing a new tool for supernova spectral modeling.

Findings

JEKYLL shows good agreement with ARTIS, SUMO, and CMFGEN in spectral and gas state calculations.

The method converges in time-dependent NLTE cases, validated by comparisons.

NLTE effects significantly influence supernova lightcurves, emphasizing the need for full NLTE modeling.

Abstract

We present JEKYLL, a new code for modelling of supernova (SN) spectra and lightcurves based on Monte-Carlo (MC) techniques for the radiative transfer. The code assumes spherical symmetry, homologous expansion and steady state for the matter, but is otherwise capable of solving the time-dependent radiative transfer problem in non-local-thermodynamic-equilibrium (NLTE). The method used was introduced in a series of papers by Lucy, but the full time-dependent NLTE capabilities of it have never been tested. Here, we have extended the method to include non-thermal excitation and ionization as well as charge-transfer and two-photon processes. Based on earlier work, the non-thermal rates are calculated by solving the Spencer-Fano equation. Using a method previously developed for the SUMO code, macroscopic mixing of the material is taken into account in a statistical sense. In addition, a statistical Markov-chain model is used to sample the emission frequency, and we introduce a method to control the sampling of the radiation field. Except for a description of JEKYLL, we provide comparisons with the ARTIS, SUMO and CMFGEN codes, which show good agreement in the calculated spectra as well as the state of the gas. In particular, the comparison with CMFGEN, which is similar in terms of physics but uses a different technique, shows that the Lucy method does indeed converge in the time-dependent NLTE case. Finally, as an example of the time-dependent NLTE capabilities of JEKYLL, we present a model of a Type IIb SN, taken from a set of models presented and discussed in detail in an accompanying paper. Based on this model we investigate the effects of NLTE, in particular those arising from non-thermal excitation and ionization, and find strong effects even on the bolometric lightcurve. This highlights the need for full NLTE calculations when simulating the spectra and lightcurves of SNe.