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We introduce the "Asymmetric Radiative Transfer In Shells Technique"(ARTIST), a new method for photon propagation on large scales that explicitlyconserves photons, propagates photons at the speed of light, approximatelyaccounts for photon directionality, and closely reproduces results of moredetailed radiative transfer (RT) codes. Crucially, it is computationally fastenough to evolve the large cosmological volumes required to predict the 21cmpower spectrum on scales that will be probed by future experiments targetingthe Epoch of Reionisation (EoR). Most semi-numerical models aimed at predictingthe 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 notaccounting for light-travel time. By implementing our RT method within thesemi-numerical code SimFast21, we show that ARTIST predicts a significantlydifferent evolution for the EoR ionisation field compared to the code's nativeESF. In particular, ARTIST predicts a more gradual evolution of thevolume-averaged ionisation fraction, and up to an order-of-magnitude differencein the ionisation power, depending on the physical parameters assumed. Itsapplication to large-scale EoR simulations will therefore allow morephysically-motivated constraints to be obtained for key EoR parameters, such asthe escape fraction.

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