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.
Categories: astro-ph.HE