The local B-polarization of the CMB: a very sensitive probe of cosmic
  defects
  We present a new and especially powerful signature of cosmic strings and
other topological or non-topological defects in the polarization of the cosmic
microwave background (CMB). We show that even if defects contribute 1% or less
in the CMB temperature anisotropy spectrum, their signature in the local
$\tilde{B}$-polarization correlation function at angular scales of tens of arc
minutes is much larger than that due to gravitational waves from inflation,
even if the latter contribute with a ratio as big as $r\simeq 0.1$ to the
temperature anisotropies. We show that when going from non-local to local
$\tilde{B}$-polarization, the ratio of the defect signal-to-noise with respect
to the inflationary value increases by about an order of magnitude. Proposed
B-polarization experiments, with a good sensitivity on arcminute scales, may
either detect a contribution from topological defects produced after inflation
or place stringent limits on them. Even Planck should be able to improve
present constraints on defect models by at least an order of magnitude, to the
level of $\ep <10^{-7}$. A future full-sky experiment like CMBpol, with
polarization sensitivities of the order of $1\mu$K-arcmin, will be able to
constrain the defect parameter $\ep=Gv^2$ to a few $\times10^{-9}$, depending
on the defect model.
