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.