Accurate Polarization Calibration at 800 MHz with the Green Bank Telescope

Polarization leakage of foreground synchrotron emission is a critical issue in H I intensity mapping experiments. While the sought-after H I emission is unpolarized, polarized foregrounds such as Galactic and extragalactic synchrotron radiation, if coupled with instrumental impurity, can mimic or overwhelm the H I signals. In this paper, we present the methodology for polarization calibration at 700-900 MHz, applied on data obtained from the Green Bank Telescope (GBT). We use astrophysical sources, both polarized and unpolarized sources including quasars and pulsars, as calibrators to characterize the polarization leakage and control systematic effects in our GBT H I intensity mapping project. The resulting fractional errors on polarization measurements on boresight are well controlled to within 0.6%-0.8% of their total intensity. The polarized beam patterns are measured by performing spider scans across both polarized quasars and pulsars. A dominant Stokes I to V leakage feature and secondary features of Stokes I to Q and I to U leakages in the 700-900 MHz frequency range are identified. These characterizations are important for separating foreground polarization leakage from the H I 21 cm signal.