Fully Polarized Calibration of HERA-19 Commissioning Data

To understand the first stars and galaxies of the early universe, we would like to measure the highly redshifted line of neutral hydrogen. The Hydrogen Epoch of Reionization Array (HERA) is a dedicated instrument that measures the power spectrum of the highly redshifted 21-cm emission line of hydrogen. It is a low-frequency radio interferometer, currently under construction in South Africa on the site of the future SKA. When complete, HERA will comprise 350 14-meter antennas operating from 50 - 225 MHz. The data here is taken from the well-characterized commissioning data of a 19-element hexagonal close-packed subarray operating from 110 - 190 MHz (as described in Kohn et al. 2019; ApJ 882:58) over seven nights. The problem we address is the necessity of calibrating polarization accurately in order to understand how polarized sources will corrupt our measurements. This requires a fully polarized calibration scheme, which is different than the standard HERA pipeline (Dillon et al. 2020, MNRAS accepted; Kern et al. 2020, ApJ 890:122). We have implemented a version of the calibration based on Smirnov & Tasse 2015; MNRAS 449:2668. We expect that this will improve the calibration of Stokes U and V, which showed evidence of poorly determined relative phases between linear polarizations (Tucker et al. 2019; AAS #235, 177.05). We have tested this calibration technique on a simulated visibilities and gains (Jones matrices) of the HERA-19 array and found convergence, but the process still needs to be streamlined. We are currently exploring applying the technique to real data and comparing it against the previous calibration scheme.