An improved estimator for non-Gaussianity in cosmic microwave background
  observations
  An improved estimator for the amplitude fnl of local-type non-Gaussianity
from the cosmic microwave background (CMB) bispectrum is discussed. The
standard estimator is constructed to be optimal in the zero-signal (i.e.,
Gaussian) limit. When applied to CMB maps which have a detectable level of
non-Gaussianity the standard estimator is no longer optimal, possibly limiting
the sensitivity of future observations to a non-Gaussian signal. Previous
studies have proposed an improved estimator by using a realization-dependent
normalization. Under the approximations of a flat sky and a vanishingly thin
last-scattering surface, these studies showed that the variance of this
improved estimator can be significantly smaller than the variance of the
standard estimator when applied to non-Gaussian CMB maps. Here this technique
is generalized to the full sky and to include the full radiation transfer
function, yielding expressions for the improved estimator that can be directly
applied to CMB maps. The ability of this estimator to reduce the variance as
compared to the standard estimator in the face of a significant non-Gaussian
signal is re-assessed using the full CMB transfer function. As a result of the
late time integrated Sachs-Wolfe effect, the performance of the improved
estimator is degraded. If CMB maps are first cleaned of the late-time ISW
effect using a tracer of foreground structure, such as a galaxy survey or a
measurement of CMB weak lensing, the new estimator does remove a majority of
the excess variance, allowing a higher significance detection of fnl.
