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The CMB Bispectrum

We use a separable mode expansion estimator with WMAP data to estimate the bispectrum for all the primary families of non-Gaussian models. We review the late-time mode expansion estimator methodology which can be applied to any non-separable primordial and CMB bispectrum model, and we demonstrate how the method can be used to reconstruct the CMB bispectrum from an observational map. We extend the previous validation of the general estimator using local map simulations. We apply the estimator to the coadded WMAP 5-year data, reconstructing the WMAP bispectrum using $l<500$ multipoles and $n=31$ orthonormal 3D eigenmodes. We constrain all popular nearly scale-invariant models, ensuring that the theoretical bispectrum is well-described by a convergent mode expansion. Constraints from the local model $ \fnl=54.4\pm 29.4$ and the equilateral model $\fnl=143.5\pm 151.2$ ($\Fnl = 25.1\pm 26.4$) are consistent with previously published results. (Here, we use a nonlinearity parameter $\Fnl$ normalised to the local case, to allow more direct comparison between different models.) Notable new constraints from our method include those for the constant model $\Fnl = 35.1 \pm 27.4 $, the flattene