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 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 $0.92 pm 0.01rm{(stat)} pm 0.07rm{(sys)}$. This
ratio corresponds to a brightness temperature of $3.8,mathrm{m K}$ 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.
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