Kavli Affiliate: Tomotake Matsumura
| First 5 Authors: Takuro Fujino, Satoru Takakura, Shahed Shayan Arani, Darcy Barron, Carlo Baccigalupi
| Summary:
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 observations of the textsc{Polarbear} project. Although the detectors are
sensitive to linear polarization, we can measure the circular polarization
because a continuously rotating half-wave plate in the optics converts part of
circular polarization into linear polarization. The atmospheric circular
polarization signal appears as a modulated signal at twice the frequency of
rotation of the half-wave plate. We reconstruct the azimuthal gradient of the
circular polarization signal and measure the dependencies on the scanning
azimuth and the detector bandpass. We compare the signal with a simulation
based on atmospheric emission theory, the detector bandpass, and the half-wave
plate leakage spectrum model. We find the ratio of the observed azimuthal slope
to the simulated slope is $0.92 pm 0.01rm{(stat)} pm 0.07rm{(sys)}$, which
demonstrates that our measurement is consistent with theoretical prediction.
This result validates our understanding of the instrument and reinforces the
feasibility of measuring the circular polarization using the imperfection of
the half-wave plate. Quantifying atmospheric circular polarization is the first
step toward conducting a search for cosmological circular polarization at these
wavelengths.
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