Measuring the reionization 21 cm fluctuations using clustering wedges
  One of the main challenges in probing the reionization epoch using the
redshifted 21 cm line is that the magnitude of the signal is several orders
smaller than the astrophysical foregrounds. One of the methods to deal with the
problem is to avoid a wedge-shaped region in the Fourier $k_{\perp} -
k_{\parallel}$ space which contains the signal from the spectrally smooth
foregrounds. However, measuring the spherically averaged power spectrum using
only modes outside this wedge (i.e., in the reionization window), leads to a
bias. We provide a prescription, based on expanding the power spectrum in terms
of the shifted Legendre polynomials, which can be used to compute the angular
moments of the power spectrum in the reionization window. The prescription
requires computation of the monopole, quadrupole and hexadecapole moments of
the power spectrum using the theoretical model under consideration and also the
knowledge of the effective extent of the foreground wedge in the $k_{\perp} -
k_{\parallel}$ plane. One can then calculate the theoretical power spectrum in
the window which can be directly compared with observations. The analysis
should have implications for avoiding any bias in the parameter constraints
using 21 cm power spectrum data.
