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The CMB bispectrum from recombination

We compute the cosmic microwave background temperature bispectrum generated by nonlinearities at recombination on all scales. We use CosmoLib$2^{\rm nd}$, a numerical Boltzmann code at second-order to compute CMB bispectra on the full sky. We consistently include all effects except gravitational lensing, which can be added to our result using standard methods. The bispectrum is peaked on squeezed triangles and agrees with the analytic approximation in the squeezed limit at the few per cent level for all the scales where this is applicable. On smaller scales, we recover previous results on perturbed recombination. For cosmic-variance limited data to $l_{\rm max} =2000$, its signal-to-noise is $S/N=0.47$ and will bias a local signal by $f_{\rm NL}^{\rm loc}\simeq 0.82$.