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