Investigation of polarized Synchrotron frequency dependence for CMB observations

Abstract

Hunting for the faint B-mode polarization of the Cosmic Microwave Background (CMB) is an open problem today since it would provide direct evidence for primordial gravitational waves. Moreover, the CMB B-mode would be a powerful probe of inflationary models, and the fundamental physics driving them at energy scales way beyond the reach of particle accelerators. Characterization and removal of contaminating foregrounds that dominate over this primordial signal is a crucial observational challenge. We investigate the diffuse Galactic synchrotron emission, which is the dominant contaminant at frequencies below 70-100 GHz. We reconstruct its synchrotron spectral index (β) map on 30% of the sky, by modelling polarization as a Rician random variable, due to noise in the measurements. We combine low-frequency WMAP and Planck polarization datasets with radio data from S-PASS, in a Bayesian framework.

We first test our method on simulations and find that it recovers unbiased estimates of the true spectral index, by using MCMC to estimate errors on β. Combining fewer datasets gives less accurate estimates, motivating the inclusion of additional datasets for improved constraints in the future, using our method. Applying to our real data, we recover a mean β = −3.21 ± 0.12. We introduce a threshold that identifies around 75% pixels with reliable estimates of β . We show that spatial variation of synchrotron polarization is a real feature beyond noise effects, and find it to be higher than reported earlier. We also calculate the power spectrum of the β map to quantify its scale dependence. More realistic simulations can be built using our results, which are crucial for testing component separation. This work can also be extended to forecast the impact of spatial variation of synchrotron polarization on its level of contamination and foreground removal, which are extremely relevant for upcoming CMB experiments.