Fractional polarization of extragalactic sources in the 500 deg² SPTpol survey

We study the polarization properties of extragalactic sources at 95 and 150 GHz in the SPTpol 500 deg² survey. We estimate the polarized power by stacking maps at known source positions, and correct for noise bias by subtracting the mean polarized power at random positions in the maps. We show that the method is unbiased using a set of simulated maps with similar noise properties to the real SPTpol maps. We find a flux-weighted mean-squared polarization fraction 〈p²〉 = [8.9 ± 1.1] × 10⁻⁴ at 95 GHz and [6.9 ± 1.1] × 10⁻⁴ at 150 GHz for the full sample. This is consistent with the values obtained for a subsample of active galactic nuclei. For dusty sources, we find 95 per cent upper limits of 〈p²〉₉₅ < 16.9 × 10⁻³and 〈p²〉₁₅₀ < 2.6 × 10⁻³. We find no evidence that the polarization fraction depends on the source flux or observing frequency. The 1σ upper limit on measured mean-squared polarization fraction at 150 GHz implies that extragalactic foregrounds will be subdominant to the CMB E and B mode polarization power spectra out to at least ℓ ≲ 5700 (ℓ ≲ 4700) and ℓ ≲ 5300 (ℓ ≲ 3600), respectively, at 95 (150) GHz.