Spin-based removal of instrumental systematics in 21cm intensity mapping
  surveys
  Upcoming cosmological intensity mapping surveys will open new windows on the
Universe, but they must first overcome a number of significant systematic
effects, including polarization leakage. We present a formalism that uses scan
strategy information to model the effect of different instrumental systematics
on the recovered cosmological intensity signal for `single-dish'
(autocorrelation) surveys. This modelling classifies different systematics
according to their spin symmetry, making it particularly relevant for dealing
with polarization leakage. We show how to use this formalism to calculate the
expected contamination from different systematics as a function of the scanning
strategy. Most importantly, we show how systematics can be disentangled from
the intensity signal based on their spin properties via map-making. We
illustrate this, using a set of toy models, for some simple instrumental
systematics, demonstrating the ability to significantly reduce the
contamination to the observed intensity signal. Crucially, unlike existing
foreground removal techniques, this approach works for signals that are
non-smooth in frequency, e.g. polarized foregrounds. These map-making
approaches are simple to apply and represent an orthogonal and complementary
approach to existing techniques for removing systematics from upcoming 21cm
intensity mapping surveys.
