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

TL;DR

This paper presents new upper limits on polarized foreground emissions at 150 MHz from GMRT observations, reducing expected contamination in Epoch of Reionization (EoR) studies and introducing novel calibration and interference removal techniques.

Contribution

The study provides the first low upper limits on polarized sky brightness at 150 MHz, improving EoR foreground modeling and introducing new polarization calibration and RFI mitigation methods.

Findings

No significant polarized emission detected at the observed field.

Upper limits on polarized foregrounds are substantially lower than previous extrapolations.

Polarized structure is weaker than expected from higher frequency data.

Abstract

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 in our observed field at mid galactic latitude (J2000 08h26m+26). We find an upper limit on the 2-dimensional angular power spectrum of diffuse polarized foregrounds of [l^2 C_l/(2 PI)]^{1/2}< 3K in frequency bins of width 1 MHz at 300<l<1000. The 3-dimensional power spectrum of polarized emission, which is most directly relevant to EoR observations, is [k^3 P_p(k)/(2 PI^2)]^{1/2}< 2K at k_perp > 0.03 h/Mpc, k < 0.1 h/Mpc. This can be compared to the expected EoR signal in total intensity of [k^3 P(k)/ (2 PI^2) ]^{1/2} ~ 10 mK. We find 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.