The Atacama Cosmology Telescope: Two-season ACTPol Extragalactic Point
  Sources and their Polarization properties
  We report on measurements of the polarization of extragalactic sources at 148
GHz made during the first two seasons of the Atacama Cosmology Telescope
Polarization (ACTPol) survey. The survey covered 680 deg$^{2}$ of the sky on
the celestial equator. Polarization measurements of 169 intensity-selected
sources brighter than 30 mJy, that are predominantly Active Galactic Nuclei,
are presented. Above a total flux of 215 mJy where the noise bias removal in
the polarization measurement is reliable, we detect 26 sources, 14 of which
have a detection of linear polarization at greater than 3$\sigma_{p}$
significance. The distribution of the fractional polarization as a function of
total source intensity is analyzed. Our result is consistent with the scenario
that the fractional polarization of our measured radio source population is
independent of total intensity down to the limits of our measurements and well
described by a Gaussian distribution with a mean fractional polarization
$p=0.028\pm$0.005 and standard deviation $\sigma_{\mathrm{p}}=0.054$, truncated
at $p=0$. Extrapolating this model for the distribution of source polarization
below the ACTPol detection threshold, we predict that one could get a clean
measure of the E-mode polarization power spectrum of the microwave background
out to $\ell=6000$ with 1 $\mu$K-arcminute maps over 10$\%$ of the sky from a
future survey. We also study the spectral energy distribution of the total and
polarized source flux densities by cross-matching with low radio frequency
catalogs. We do not find any correlation between the spectral indices for total
flux and polarized flux.
