Impact of instrumental systematics on the CMB bispectrum

Abstract

We study the effects of instrumental systematics on the estimation of primordial non-Gaussianity using the cosmic microwave background (CMB) bispectrum from both the temperature and the polarization anisotropies. For temperature systematics we consider gain fluctuation and beam distortions. For polarization we consider effects related to known instrumental systematics: calibration, pixel rotation, differential gain, pointing, and ellipticity of the instrument beam. We consider these effects at the next to leading order, which we refer to as nonlinear systematic effects. We find that if the instrumental response is linearly proportional to the received CMB intensity, then only the shape of the primordial CMB bispectrum, if there is any, will be distorted. We show that the nonlinear response of the instrument can in general result in spurious non-Gaussian features on both the CMB temperature and polarization anisotropies, even if the primordial CMB is completely Gaussian. We determine the level for both the linear and nonlinear systematics parameters for which they would cause no significant degradation of our ability to constrain the primordial non-Gaussianity amplitude ${f}_{\mathrm{NL}}$. We find that the nonlinear systematics are potentially a bigger worry for extracting the primordial non-Gaussianity than the linear systematics, especially because the current and near future CMB probes are optimized for CMB power-spectrum measurements which are not particularly sensitive to the nonlinear instrument response. We find that if instrumental nonlinearities are not controlled by dedicated calibration, the effective local non-Gaussianity can be as large as ${f}_{\mathrm{NL}}\ensuremath{\sim}O(10)$ before the corresponding nonlinearities show up in the CMB dipole measurements. The higher-order multipoles are even less sensitive to instrumental nonlinearities.