Non-parametric foreground subtraction for 21cm epoch of reionization
  experiments
  An obstacle to the detection of redshifted 21cm emission from the epoch of
reionization (EoR) is the presence of foregrounds which exceed the cosmological
signal in intensity by orders of magnitude. We argue that in principle it would
be better to fit the foregrounds non-parametrically - allowing the data to
determine their shape - rather than selecting some functional form in advance
and then fitting its parameters. Non-parametric fits often suffer from other
problems, however. We discuss these before suggesting a non-parametric method,
Wp smoothing, which seems to avoid some of them. After outlining the principles
of Wp smoothing we describe an algorithm used to implement it. We then apply Wp
smoothing to a synthetic data cube for the LOFAR EoR experiment. The
performance of Wp smoothing, measured by the extent to which it is able to
recover the variance of the cosmological signal and to which it avoids leakage
of power from the foregrounds, is compared to that of a parametric fit, and to
another non-parametric method (smoothing splines). We find that Wp smoothing is
superior to smoothing splines for our application, and is competitive with
parametric methods even though in the latter case we may choose the functional
form of the fit with advance knowledge of the simulated foregrounds. Finally,
we discuss how the quality of the fit is affected by the frequency resolution
and range, by the characteristics of the cosmological signal and by edge
effects.
