Removing the ISW-lensing bias from the local-form primordial
  non-Gaussianity estimation
  The Integrated Sachs-Wolfe (ISW) effect produces a secondary temperature
anisotropy of CMB. The main contribution comes from z<2, where dark energy
leads to a decay of potentials. As the same photons are gravitationally lensed
by these decaying potentials, there exists a high degree of correlation between
the ISW effect and CMB lensing, leading to a non-zero three-point correlation
(bispectrum) of the observed temperature anisotropy. This ISW-lensing
bispectrum, whose shape resembles that of the so-called "local-form" primordial
bispectrum parametrized by fNL, is known to be the largest contamination of
fNL. In order to avoid a spurious detection of primordial non-Gaussianity, we
need to remove the ISW-lensing bias. In this work, we investigate three
debiasing methods: (I) subtraction of an expected, ensemble average of the
ISW-lensing bispectrum; (II) subtraction of a measured ISW-lensing bispectrum;
and (III) direct subtraction of an estimated ISW signal from an observed
temperature map. One may use an estimation of the ISW map from external non-CMB
data or that from the CMB data themselves. As the methods II and III are based
on fewer assumptions about the nature of dark energy, they are preferred over
the method I. While the methods I and II yield unbiased estimates of fNL with
comparable error bars, the method III yields a biased result when the
underlying primordial fNL is non-zero and the ISW map is estimated from a
lensing potential reconstructed from the observed temperature map. One of the
sources of the bias is a lensing reconstruction noise bias which is independent
of fNL and can be calculated precisely, but other fNL-dependent terms are
difficult to compute reliably. We thus conclude that the method II is the best,
model-independent way to remove the ISW-lensing bias of fNL, enabling us to
test the physics of inflation with smaller systematic errors.
