The Covariance of Squeezed Bispectrum Configurations
  We measure the halo bispectrum covariance in a large set of N-body
simulations and compare it with theoretical expectations. We find a large
correlation among (even mildly) squeezed halo bispectrum configurations. A
similarly large correlation can be found between squeezed triangles and the
long-wavelength halo power spectrum. This shows that the diagonal Gaussian
contribution fails to describe, even approximately, the full covariance in
these cases. We compare our numerical estimate with a model that includes, in
addition to the Gaussian one, only the non-Gaussian terms that are large for
squeezed configurations. We find that accounting for these large terms in the
modeling greatly improves the agreement of the full covariance with
simulations. We apply these results to a simple Fisher matrix forecast, and
find that constraints on primordial non-Gaussianity are degraded by a factor of
$\sim 2$ when a non-Gaussian covariance is assumed instead of the diagonal,
Gaussian approximation.
