Sensitivity-Improved Polarization Maps at 40 GHz with CLASS and WMAP data

TL;DR

This paper enhances 40 GHz polarization maps by combining CLASS and WMAP data, reducing noise and improving large-scale measurements to better study Galactic synchrotron radiation and primordial B-modes.

Contribution

It introduces a novel combination of CLASS and WMAP data in the harmonic domain, significantly improving sensitivity and large-scale polarization measurements at 40 GHz.

Findings

Noise level reduced by a factor of 2-3 in the intermediate multipole range.

Polarized synchrotron spectral index shows significant spatial variation.

Combined maps exhibit negligible bias in power spectra, validated by simulations.

Abstract

Improved polarization measurements at frequencies below 70 GHz with degree-level angular resolution are crucial for advancing our understanding of the Galactic synchrotron radiation and the potential polarized anomalous microwave emission and ultimately benefiting the detection of primordial $B$ modes. In this study, we present sensitivity-improved 40 GHz polarization maps obtained by combining the CLASS 40 GHz and WMAP $Q$-band data through a weighted average in the harmonic domain. The decision to include WMAP $Q$-band data stems from similarities in the bandpasses. Leveraging the accurate large-scale measurements from WMAP $Q$ band and the high-sensitivity information from CLASS 40 GHz band at intermediate scales, the noise level at $\ell\in[30, 100]$ is reduced by a factor of $2-3$ in the map space. A pixel domain analysis of the polarized synchrotron spectral index ($\beta_s$) using WMAP $K$ band and the combined maps (mean and 16/84th percentile across the $\beta_s$ map: $-3.08_{-0.20}^{+0.20}$) reveals a stronger preference for spatial variation (PTE for a uniform $\beta_s$ hypothesis smaller than 0.001) than the results obtained using WMAP $K$ and $Ka$ bands ($-3.08_{-0.14}^{+0.14}$). The cross-power spectra of the combined maps follow the same trend as other low-frequency data, and validation through simulations indicates negligible bias introduced by the combination method (sub-percent level in the power spectra). The products of this work are publicly available on $\mathtt{LAMBDA}$.