Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A Measurement of Circular Polarization at 40 GHz

TL;DR

This paper reports the first measurements of circular polarization at 40 GHz from the CLASS experiment, detecting atmospheric emission and setting new upper limits on CMB circular polarization, advancing understanding of foregrounds and polarization signals.

Contribution

It provides the first detection of atmospheric circular polarization at 40 GHz and improves constraints on CMB circular polarization by two orders of magnitude.

Findings

Detected atmospheric circular polarization with a dipole pattern at 124 μK.

Placed upper limits of 0.4 μK^2 to 13.5 μK^2 on CMB circular polarization.

Demonstrated the effectiveness of the Variable-delay Polarization Modulator for polarization measurements.

Abstract

We report circular polarization measurements from the first two years of observation with the 40 GHz polarimeter of the Cosmology Large Angular Scale Surveyor (CLASS). CLASS is conducting a multi-frequency survey covering 75% of the sky from the Atacama Desert designed to measure the cosmic microwave background (CMB) linear E and B polarization on angular scales $1^\circ \lesssim \theta \leq 90^\circ$, corresponding to a multipole range of $2 \leq \ell \lesssim 200$. The modulation technology enabling measurements of linear polarization at the largest angular scales from the ground, the Variable-delay Polarization Modulator, is uniquely designed to provide explicit sensitivity to circular polarization (Stokes $V$). We present a first detection of circularly polarized atmospheric emission at 40 GHz that is well described by a dipole with an amplitude of $124\pm4\,\mathrm{\mu K}$ when observed at an elevation of $45^\circ$, and discuss its potential impact as a foreground to CMB experiments. Filtering the atmospheric component, CLASS places a 95% C.L. upper limit of $0.4\,\mathrm{\mu K}^2$ to $13.5\,\mathrm{\mu K}^2$ on $\ell(\ell+1)C_\ell^{VV}/(2\pi)$ between $1 \leq \ell \leq 120$, representing a two-orders-of-magnitude improvement over previous limits.