A hybrid map-C? component separation method for primordial CMB B-mode searches

The observation of the polarised emission from the Cosmic Microwave Background (CMB) from future ground-based and satellite-borne experiments holds the promise of indi-rectly detecting the elusive signal from primordial tensor fluctuations in the form of large-scale B-mode polarisation. Doing so, however, requires an accurate and robust separation of the sig-nal from polarised Galactic foregrounds. We present a component separation method for multi -frequency CMB observations that combines some of the advantages of map-based and power -spectrum-based techniques, and which is direcly applicable to data in the presence of realistic foregrounds and instrumental noise. We demonstrate that the method is able to reduce the con-tamination from Galactic foregrounds below an equivalent tensor-to-scalar ratio rFG <= 5x10-4, as required for next-generation observatories, for a wide range of foreground models with varying degrees of complexity. This bias reduction is associated with a mild -20-30% increase in the final statistical uncertainties, and holds for large sky areas, and for experiments targeting both the reionisation and recombination bumps in the B-mode power spectrum.