A measurement of atmospheric circular polarization with POLARBEAR

Abstract

At millimeter wavelengths, the atmospheric emission is circularly polarized owing to the Zeeman splitting of molecular oxygen by the Earth's magnetic field. We report a measurement of the signal in the 150 GHz band using 3 years of observational data with the \textsc{Polarbear} project. Non-idealities of a continuously rotating half-wave plate (HWP) partially convert circularly polarized light to linearly polarized light. While \textsc{Polarbear} detectors are sensitive to linear polarization, this effect makes them sensitive to circular polarization. Although this was not the intended use, we utilized this conversion to measure circular polarization. We reconstruct the azimuthal gradient of the circular polarization signal and measure its dependency from the scanning direction and the detector bandpass. We compare the signal with a simulation based on atmospheric emission theory, the detector bandpass, and the HWP leakage spectrum model. We find the ratio of the observed azimuthal slope to the simulated slope is $0.92\pm 0.01(\mathrm{stat})\pm 0.07(\mathrm{sys})$. This ratio corresponds to a brightness temperature of $3.8\mathrm{mK}$ at the effective band center of $121.8\mathrm{GHz}$ and bandwidth of $3.5\mathrm{GHz}$ estimated from representative detector bandpass and the spectrum of Zeeman emission. This result validates our understanding of the instrument and reinforces the feasibility of measuring the circular polarization using the imperfection of continuously rotating HWP. Continuously rotating HWP is popular in ongoing and future cosmic microwave background experiments to modulate the polarized signal. This work shows a method for signal extraction and leakage subtraction that can help measuring circular polarization in such experiments.

23 pages, 11 figures

References in corpus (9)

• Rotation of Linear Polarization Plane and Circular Polarization from Cosmological Pseudoscalar Fields
• Design and development of an ambient-temperature continuously-rotating achromatic half-wave plate for CMB polarization modulation on the POLARBEAR-2 experiment
• Improved upper limit on degree-scale CMB B-mode polarization power from the 670 square-degree POLARBEAR survey
• Probing Lorentz-violating electrodynamics with CMB polarization
• Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A Measurement of Circular Polarization at 40 GHz
• Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A First Detection of Atmospheric Circular Polarization at Q Band
• CMB $V$ modes from photon-photon forward scattering revisited
• The Simons Observatory: Development and Optical Evaluation of Achromatic Half-Wave Plates
• Characterization of a half-wave plate for cosmic microwave background circular polarization measurement with POLARBEAR