21CMMC with a 3D light-cone: the impact of the co-evolution
  approximation on the astrophysics of reionisation and cosmic dawn
  We extend 21CMMC, a Monte Carlo Markov Chain sampler of 3D reionisation
simulations, to perform parameter estimation directly on 3D light-cones of the
cosmic 21cm signal. This brings theoretical analysis closer to the tomographic
21-cm observations achievable with next generation interferometers like HERA
and the SKA. Parameter recovery can therefore account for modes which evolve
with redshift/frequency. Additionally, simulated data can be more easily
corrupted to resemble real data. Using the light-cone version of 21CMMC, we
quantify the biases in the recovered astrophysical parameters if we use the
21cm power spectrum from the co-evolution approximation to fit a 3D light-cone
mock observation. While ignoring the light-cone effect under most assumptions
will not significantly bias the recovered astrophysical parameters, it can lead
to an underestimation of the associated uncertainty. However significant biases
($\sim$few -- 10 $\sigma$) can occur if the 21cm signal evolves rapidly (i.e.
the epochs of reionisation and heating overlap significantly) and: (i)
foreground removal is very efficient, allowing large physical scales
($k\lesssim0.1$~Mpc$^{-1}$) to be used in the analysis or (ii) theoretical
modelling is accurate to within $\sim10$ per cent in the power spectrum
amplitude.
