Speaker
Description
Primordial gravitational waves from inflation imprint a faint B-mode polarisation in the Cosmic Microwave Background (CMB), but this signal is strongly obscured by Galactic foregrounds. Detecting CMB B-modes therefore requires accurate characterisation of foreground polarisation, dominated by synchrotron emission at low frequencies. To address this, we apply the Generalised Needlet Internal Linear Combination (GNILC) method to Planck PR4 data (30–353 GHz), producing clean, low-noise synchrotron polarisation maps at low frequencies without relying on prior models. GNILC employs needlets for localised processing in both pixel and harmonic space, a principal component analysis to retain modes above noise, and a multi-frequency weighting scheme orthogonal to the CMB spectrum to deproject the CMB. Radio source inpainting prior to GNILC processing further reduces contamination in the resulting maps. The method is validated on Planck NPIPE simulations, and PR4-derived GNILC weights are applied to NPIPE noise, systematics and CMB realizations to provide robust residual error characterisation of the GNILC PR4 maps. The final GNILC PR4 maps at 30 and 44 GHz provide reliable full-sky diffuse polarised synchrotron templates at uniform 34' angular resolution, as demonstrated by map inspection, residual error estimates, power spectra, and low correlation with Planck cleaned CMB maps. Exploiting the frequency scaling between our 30 and 44 GHz GNILC PR4 maps, we derive a first-of-its-kind 48-pixel full sky synchrotron spectral index map from Planck-only data, with the associated uncertainty.