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Submillimetre polarization of the Crab Nebula Rodrigues, Daniel


Observations of the cosmic microwave background (CMB) have been fundamental in cosmology for more than 50 years. Many discoveries have been enabled by groundbreaking CMB experiments, including the consolidation of the big bang theory and the determination of the main parameters that describe our Universe. However, there is still more that we can learn from the CMB. In particular, there is the potential to better understand whether the early Universe underwent a period of inflation. To do this we need higher precision measurements of CMB polarization, particularly to detect this so-called 𝐵 modes. Measuring 𝐵 modes is extremely hard for several different reasons, such as instrumental challenges, foreground contamination and weak signal. B-mode experiments, therefore, must be calibrated extraordinarily well. One possible calibration method is to use known sources in the sky as polarization references. The goal of this study is to provide improved calibration source data, at a wavelength of 850𝝻m. This could help future CMB experiments, including the Japanese-led Lite (Light) satellite for the studies of B-mode polarization and Inflation from cosmic background Radiation Detection (LiteBIRD). We therefore analyse the submillimetre polarization properties of the Crab Nebula, which is the brightest polarized object in the sky at submillimetre wavelengths. We would like to determine its polarization angle to a precision of 0.1° for 850µm light, to fulfill the requirements for CMB 𝐵 modes. We use data from the James Clerk Maxwell Telescope (JCMT) and its Submillimeter Common-User Bolometer Array 2 (SCUBA-2) 10,240-pixel bolometer camera. We discuss the details of the data reduction using JCMT's software Starlink, and reach the final value of 141(±16)°, in equatorial coordinates, for the polarization angle of the Crab Nebula. We see large variations in the polarization angle between subsets of the data, indicating that the measurement is dominated by systematic uncertainties (rather than statistical). Although we recognize that there may still be improvements to be made in the reduction process, our conclusion is that it will be extremely challenging to achieve the desired precision.

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