Mitigating half-wave plate systematics at the map-making level: calibration requirements for LiteBIRD

TL;DR

This paper analyzes how inaccuracies in characterizing half-wave plate non-idealities can bias the measurement of the tensor-to-scalar ratio in CMB experiments, and establishes calibration requirements for LiteBIRD.

Contribution

It introduces an end-to-end framework to assess the impact of HWP non-ideality discrepancies on CMB measurements and derives calibration requirements for LiteBIRD.

Findings

Discrepancies in HWP profiles can bias the tensor-to-scalar ratio measurement.

Calibration requirements are established to prevent systematic biases.

Results are robust across different sky models.

Abstract

Although half-wave plates (HWPs) are becoming a popular choice of polarization modulators for cosmic microwave background (CMB) experiments, their non-idealities can introduce systematic effects that should be carefully characterized and mitigated. One possible mitigation strategy is to incorporate information about the non-idealities at the map-making level, which helps to reduce the HWP-induced distortions of the reconstructed CMB. Nevertheless, the non-idealities can only be known with finite precision. In this paper we investigate the consequences of discrepancies between their true frequency profiles and those assumed by the map-maker. We present an end-to-end framework, including a blind component-separation step, and use it to translate these discrepancies into a bias on the tensor-to-scalar ratio, $r$, for the LiteBIRD satellite mission. We subsequently derive realistic and conservative measurement requirements for accurately characterizing the HWP non-idealities to ensure they do not compromise LiteBIRD's ambitious scientific goals. We find that the obtained results are robust against sky models with varying complexity.