Non-Gaussianity and the CMB Bispectrum: confusion between Primordial and
  Lensing-Rees Sciama contribution?
  We revisit the predictions for the expected Cosmic Microwave Background
bispectrum signal from the primary-lensing-Rees-Sciama correlation; we point
out that it can be a significant contaminant to the bispectrum signal from
primordial non-Gaussianity of the local type. This non-Gaussianity, usually
parameterized by the non-Gaussian parameter f_NL, arises, for example, in
multi-field inflation. In particular both signals are frequency independent,
and are maximized for nearly squeezed configurations. While their detailed
scale-dependence and harmonic imprints are different for generic bispectrum
shapes, we show that, if not included in the modeling, the
primary-lensing-Rees-Sciama contribution yields an effective f_{NL} of 10 when
using a bispectrum estimator optimized for local non-Gaussianity. Considering
that expected 1-sigma errors on f_{NL} are < 10 from forthcoming experiments,
we conclude that the contribution from this signal must be included in future
constraints on f_{NL} from the Cosmic Microwave Background bispectrum.
