# POSSIS: predicting spectra, light curves and polarization for multi-dimensional models of supernovae and kilonovae

## TL;DR

POSSIS is a 3D Monte Carlo code that models radiation transport in supernovae and kilonovae, predicting spectra, light curves, and polarization with a focus on multi-dimensional effects and parameter studies.

## Contribution

The paper introduces POSSIS, a novel 3D Monte Carlo code that incorporates wavelength- and time-dependent opacities for detailed modeling of supernovae and kilonovae.

## Key findings

- A kilonova model with specific ejecta parameters matches GW 170817 observations.
- Multi-dimensional models are essential over 1D models for accurate parameter inference.
- POSSIS enables fast parameter-space exploration of supernovae and kilonovae.

## Abstract

We present POSSIS, a time-dependent three-dimensional Monte Carlo code for modelling radiation transport in supernovae and kilonovae. The code incorporates wavelength- and time-dependent opacities and predicts viewing-angle dependent spectra, light curves and polarization for both idealized and hydrodynamical explosion models. We apply the code to a kilonova model with two distinct ejecta components, one including lanthanide elements with relatively high opacities and the other devoid of lanthanides and characterized by lower opacities. We find that a model with total ejecta mass $M_\mathrm{ej}=0.04\,M_\odot$ and half-opening angle of the lanthanide-rich component $\Phi=30^\circ$ provides a good match to GW 170817 / AT 2017gfo for orientations near the polar axis (i.e. for a system viewed close to face-on). We then show how crucial is the use of self-consistent multi-dimensional models in place of combining one-dimensional models to infer important parameters as the ejecta masses. We finally explore the impact of $M_\mathrm{ej}$ and $\Phi$ on the synthetic observables and highlight how the relatively fast computation times of POSSIS make it well-suited to perform parameter-space studies and extract key properties of supernovae and kilonovae. Spectra calculated with POSSIS in this and future studies will be made publicly available.