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

We report measurements of circular polarization 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° ≲ θ ≤ 90°, corresponding to a multipole range of 2 ≤ ℓ ≲ 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 $124\pm 4\,\mu {\rm{K}}$ when observed at an elevation of 45°, and discuss its potential impact on the recovery of linear polarization by CLASS. Filtering the atmospheric component, CLASS places a 95% confidence upper limit of $0.4\,\mu {{\rm{K}}}^{2}$ to $13.5\,\mu {{\rm{K}}}^{2}$ on ${\ell }({\ell }+1){C}_{{\ell }}^{{VV}}/(2\pi )$ for $1\leqslant {\ell }\leqslant 120$ , representing an improvement by two orders of magnitude over previous CMB limits.