# ARTIST: Fast radiative transfer for large-scale simulations of the epoch of reionisation

## TL;DR

ARTIST is a new, fast radiative transfer method that conserves photons and accurately models the Epoch of Reionisation, enabling more realistic large-scale simulations for upcoming 21cm observations.

## Contribution

The paper introduces ARTIST, a novel radiative transfer technique that improves accuracy and speed over existing semi-numerical models for simulating the Epoch of Reionisation.

## Key findings

- ARTIST predicts a more gradual ionisation evolution.
- It shows up to an order-of-magnitude difference in ionisation power.
- The method enables physically-motivated constraints on EoR parameters.

## Abstract

We introduce the "Asymmetric Radiative Transfer In Shells Technique" (ARTIST), a new method for photon propagation on large scales that explicitly conserves photons, propagates photons at the speed of light, approximately accounts for photon directionality, and closely reproduces results of more detailed radiative transfer (RT) codes. Crucially, it is computationally fast enough to evolve the large cosmological volumes required to predict the 21cm power spectrum on scales that will be probed by future experiments targeting the Epoch of Reionisation (EoR). Most semi-numerical models aimed at predicting the EoR 21cm signal make use of an excursion set formalism (ESF) approach, which achieves computational viability by compromising on photon conservation, constraining ionised regions to be spherical by construction, and not accounting for light-travel time. By implementing our RT method within the semi-numerical code SimFast21, we show that ARTIST predicts a significantly different evolution for the EoR ionisation field compared to the code's native ESF. In particular, ARTIST predicts a more gradual evolution of the volume-averaged ionisation fraction, and up to an order-of-magnitude difference in the ionisation power, depending on the physical parameters assumed. Its application to large-scale EoR simulations will therefore allow more physically-motivated constraints to be obtained for key EoR parameters, such as the escape fraction.