ExTraSS: a Domain Decomposed 3D NLTE Radiative Transfer spectral synthesis code for nebular phase transients

TL;DR

ExTraSS is a new 3D NLTE radiative transfer code for nebular phase supernova spectra, featuring a domain decomposition algorithm to handle complex computations efficiently.

Contribution

The paper introduces ExTraSS, a fully capable 3D NLTE radiative transfer code with a novel domain decomposition method for supernova spectral modeling.

Findings

Successfully manages millions of photoexcitation rates across 10^5 cells.

Demonstrates convergence and accuracy in 3D NLTE radiative transfer calculations.

Enables synthetic spectra generation for asymmetric supernova ejecta.

Abstract

In the nebular phase, supernovae are powered by radioactive decay and continuously fade, while their densities have decreased enough such that the expanding nebula becomes (largely) optically thin and the entire structure contributes to the emission. Models for the nebular phase need to take Non-Local Thermodynamic Equilibrium (NLTE) effects into account, while at the same time radiative transfer effects often cannot be ignored. To account for the asymmetric morphologies of SNe, 3D input ejecta models must be used. In this work, we present the $\texttt{ExTraSS}$ (EXplosive TRAnsient Spectral Simulator) code, which has been upgraded to be fully capable of 3D NLTE radiative transfer calculations in order to generate synthetic spectra for explosive transients in the nebular phase, with a focus on supernovae. We solve a long-standing difficulty of 3D NLTE radiative transfer -- to manage generation and storage of millions of photoexcitation rates over $\gtrsim10^{5}$ of cells -- by developing a new Domain Decomposition algorithm. We describe this new methodology and general code operations in detail, and analyse convergence and accuracy for $\texttt{ExTraSS}$.