Modification of the 21-cm power spectrum by quasars during the epoch of
  reionisation
  We assess the effect of a population of high-redshift quasars on the 21-cm
power spectrum during the epoch of reionisation. Our approach is to implement a
semi-numerical scheme to calculate the three-dimensional structure of ionised
regions surrounding massive halos at high redshift. We include the ionising
influence of luminous quasars by populating a simulated overdensity field with
quasars using a Monte Carlo Markov Chain algorithm. We find that quasars modify
both the amplitude and shape of the power spectrum at a level which is of the
same order as the fractional contribution to reionisation. The modification is
found both at constant redshift and at constant global neutral fraction, and
arises because ionising photons produced by quasars are biased relative to the
density field at a level that is higher than steller ionising photons. Our
results imply that quasar ionisation will need to be included in detailed
modelling of observed 21-cm power spectra.
