CLASS Angular Power Spectra and Map-Component Analysis for 40 GHz Observations through 2022

TL;DR

This paper presents detailed analysis of 40 GHz CMB polarization data from the CLASS experiment, achieving high sensitivity and new constraints on polarization spectra, including limits on circular polarization, enhancing ground-based measurements of large-scale CMB features.

Contribution

The study introduces improved ground-based measurements of large-scale CMB polarization at 40 GHz, including new limits on circular polarization and expanded synchrotron spectral energy density analysis.

Findings

Higher sensitivity than satellite measurements in the 10 < ℓ < 100 range.

New upper limit on circular polarization background, D_ℓ < 0.023 μK² at 95% confidence.

Expanded measurements of synchrotron spectral energy density across new sky regions.

Abstract

Measurement of the largest angular scale ($\ell < 30$) features of the cosmic microwave background (CMB) polarization is a powerful way to constrain the optical depth to reionization and search for the signature of inflation through the detection of primordial $B$-modes. We present an analysis of maps covering 73.6\% of the sky made from the $40\,\mathrm{GHz}$ channel of the Cosmology Large Angular Scale Surveyor (CLASS) from 2016 August to 2022 May. Taking advantage of the measurement stability enabled by front-end polarization modulation and excellent conditions from the Atacama Desert, we show this channel achieves higher sensitivity than the analogous frequencies from satellite measurements in the range $10 < \ell < 100$. Simulations show the CLASS linear (circular) polarization maps have a white noise level of $125 \,(130)\,\mathrm{\mu K\, arcmin}$. We measure the Galaxy-masked $EE$ and $BB$ spectra of diffuse synchrotron radiation and compare to space-based measurements at similar frequencies. In combination with external data, we expand measurements of the spatial variations of the synchrotron spectral energy density (SED) to include new sky regions and measure the diffuse SED in the harmonic domain. We place a new upper limit on a background of circular polarization in the range $5 < \ell < 125$ with the first bin showing $D_\ell < 0.023$ $\mathrm{\mu K^2_{CMB}}$ at 95\% confidence. These results establish a new standard for recovery of the largest-scale CMB polarization from the ground and signal exciting possibilities when the higher sensitivity and higher-frequency CLASS channels are included in the analysis.