Long-Timescale Stability in CMB Observations at Multiple Frequencies using Front-End Polarization Modulation

TL;DR

This paper demonstrates that front-end polarization modulation significantly improves the long-term stability of CMB polarization measurements across multiple frequencies, enabling more precise cosmological observations.

Contribution

It presents a detailed analysis of stability improvements in CMB observations achieved through front-end polarization modulation across four frequency bands.

Findings

Median knee frequencies reduced by an order of magnitude with modulation.

Stable polarization measurements achieved over one month of data.

Enhanced long-term stability supports better constraints on cosmological parameters.

Abstract

The Cosmology Large Angular Scale Surveyor (CLASS) is a telescope array observing the Cosmic Microwave Background (CMB) at frequency bands centered near 40, 90, 150, and 220 GHz. CLASS measures the CMB polarization on the largest angular scales to constrain the inflationary tensor-to-scalar ratio and the optical depth due to reionization. To achieve the long time-scale stability necessary for this measurement from the ground, CLASS utilizes a front-end, variable-delay polarization modulator on each telescope. Here we report on the improvements in stability afforded by front-end modulation using data across all four CLASS frequencies. Across one month of modulated linear polarization data in 2021, CLASS achieved median knee frequencies of 9.1, 29.1, 20.4, and 36.4 mHz for the 40, 90, 150, and 220 GHz observing bands. The knee frequencies are approximately an order of magnitude lower than achieved via CLASS pair-differencing orthogonal detector pairs without modulation.