Impact of Instrumental Systematics on the CMB Bispectrum

TL;DR

This paper investigates how instrumental systematics, especially non-linear responses, impact the estimation of primordial non-Gaussianity in the CMB bispectrum, highlighting potential biases and the importance of calibration.

Contribution

It provides a detailed analysis of linear and non-linear instrumental effects on CMB bispectrum measurements and assesses their impact on primordial non-Gaussianity constraints.

Findings

Non-linear instrument responses can induce spurious non-Gaussian features.

Linear responses mainly distort the bispectrum shape without biasing f_{nl}.

Non-linearities could mimic f_{nl} as large as 10 if not properly calibrated.

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 next to leading order, which we refer to as non-linear 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 non-linear systematics parameters for which they would cause no significant degradation of our ability to constrain the primordial non-Gaussianity amplitude f_{nl}. We find that the non-linear systematics are potentially 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 non-linear instrument response. We find that if instrumental non-linearities are not controlled by dedicated calibration, the effective local non-Gaussianity can be as large as f_{nl} ~ O(10) before the corresponding non-linearities show up in the CMB dipole measurements. The higher order multipoles are even less sensitive to instrumental non-linearities.