Impact of modelling foreground uncertainties on future CMB polarization satellite experiments

We present an analysis of errors on the tensor-to-scalar ratio due to residual diffuse foregrounds. We use simulated observations of a cosmic microwave background (CMB) polarization satellite, the Cosmic Origins Explorer (COrE), using the specifications of the version proposed to ESA in 2010. We construct a full pipeline from microwave sky maps to r likelihood, using two models of diffuse Galactic foregrounds with different complexity and assuming component separation with varying degrees of accuracy. Our pipeline uses a linear mixture (generalized least squares) solution for component separation, and a hybrid approach for power spectrum estimation, with a quadratic maximum likelihood estimator at low ls and a pseudo-Cl deconvolution at high ls. In the likelihood for r, we explore modelling foreground residuals as nuisance parameters. Our analysis aims at measuring the bias introduced in r by mismodelling the foregrounds and to determine what error is tolerable while still successfully detecting r. We find that r = 0.01 can be measured successfully even for a complex sky model and in the presence of foreground parameters error. However, the detection of r = 0.001 is a lot more challenging, as inaccurate modelling of the foreground spectral properties may result in a biased measurement of r. Once biases are eliminated, the total error on r allows setting an upper limit rather than a detection, unless the uncertainties on the foreground spectral indices are very small, i.e. equal or better than 0.5 per cent error for both dust and synchrotron. This emphasizes the need for pursuing research on component separation and foreground characterization in view of next-generation CMB polarization experiments.