The GMRT EoR Experiment: Limits on Polarized Sky Brightness at 150 MHz

The GMRT reionization effort aims to map out the large scale structure of the Universe during the epoch of reionization (EoR). Removal of polarized Galactic emission is a difficult part of any 21 cm EoR program, and we present new upper limits to diffuse polarized foregrounds at 150 MHz. We find no high significance evidence of polarized emission from diffuse or point sources in our observed field at mid galactic latitude. This places upper bounds on polarization of extragalactic point sources to < 1%. We find that diffuse polarized foregrounds are less than ~1 K in the range 300 < l < 1000. At higher rotation measures |RM| ~ 14 rad/m$^2$, the range relevant for EoR, we find 2-D power levels below ~0.2 K after summing of $\delta\nu=16$ MHz bandwidth. The constraint to a 3-D power measurement is a few K. In these units, the EoR signal is ~10 mK. We find polarized polarized structure is substantially weaker than suggested by extrapolation from higher frequency observations, so the new low upper limits reported here reduce the anticipated impact of these foregrounds on EoR experiments. We discuss Faraday beam and depth depolarization models and compare predictions of these models to our data. We report on a new technique for polarization calibration using pulsars, as well as a new technique to remove broadband radio frequency interference. Our data indicate that, on the edges of the main beam at GMRT, polarization squint creates ~3% leakage of unpolarized power into polarized maps at zero rotation measure. Ionospheric rotation was largely stable during these solar minimum night time observations.