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Observations of the redshifted 21-cm signal emitted by neutral hydrogenrepresent a promising probe of large-scale structure in the universe. However,cosmological 21-cm signal is challenging to observe due to astrophysicalforegrounds which are several orders of magnitude brighter. Traditional linearforeground removal methods can optimally remove foregrounds for a knowntelescope response but are sensitive to telescope systematic errors such asantenna gain and delay errors, leaving foreground contamination in therecovered signal. Non-linear methods such as principal component analysis, onthe other hand, have been used successfully for foreground removal, but theylead to signal loss that is difficult to characterize and requires carefulanalysis. In this paper, we present a systematics-robust foreground removaltechnique which combines both linear and non-linear methods. We first obtainsignal and foreground estimates using a linear filter. Under the assumptionthat the signal estimate is contaminated by foreground residuals induced byparameterizable systematic effects, we infer the systematics-inducedcontamination by cross-correlating the initial signal and foreground estimates.Correcting for the inferred error, we are able to subtract foregroundcontamination from the linearly filtered signal up to the first order in theamplitude of the telescope systematics. In simulations of an interferometric21-cm survey, our algorithm removes foreground leakage induced by complex gainerrors by one to two orders of magnitude in the power spectrum. Our techniquethus eases the requirements on telescope characterization for modern andnext-generation 21-cm cosmology experiments.

Removing systematics-induced 21-cm foreground residuals by cross-correlating filtered data