Fractional Polarisation of Extragalactic Sources in the
  500-square-degree SPTpol Survey
  We study the polarisation properties of extragalactic sources at 95 and 150
GHz in the SPTpol 500 deg$^2$ survey. We estimate the polarised power by
stacking maps at known source positions, and correct for noise bias by
subtracting the mean polarised 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
polarisation fraction $\langle p^2 \rangle= [8.9\pm1.1] \times 10^{-4}$ at 95
GHz and $[6.9\pm1.1] \times 10^{-4}$ at 150~GHz for the full sample. This is
consistent with the values obtained for a sub-sample of active galactic nuclei.
For dusty sources, we find 95 per cent upper limits of $\langle p^2
\rangle_{\rm 95}<16.9 \times 10^{-3}$ and $\langle p^2 \rangle_{\rm 150}<2.6
\times 10^{-3}$. We find no evidence that the polarisation fraction depends on
the source flux or observing frequency. The 1-$\sigma$ upper limit on measured
mean squared polarisation fraction at 150 GHz implies that extragalactic
foregrounds will be subdominant to the CMB E and B mode polarisation power
spectra out to at least $\ell\lesssim5700$ ($\ell\lesssim4700$) and
$\ell\lesssim5300$ ($\ell\lesssim3600$), respectively at 95 (150) GHz.
