Making maps of Cosmic Microwave Background polarization for B-mode studies: the POLARBEAR example

TL;DR

This paper develops a general filtering and map-making method for CMB polarization data, specifically targeting B-mode studies, and evaluates its effectiveness using realistic simulations based on the POLARBEAR experiment.

Contribution

It introduces a mathematically rigorous filtering approach that unambiguously removes unwanted modes and incorporates this into the map-making process for CMB polarization data.

Findings

The proposed method can produce unbiased sky maps under realistic conditions.

Filtering affects noise correlations and the performance of power spectrum estimators.

Simplified map-makers may be more practical despite the advanced method's fidelity.

Abstract

Analysis of cosmic microwave background (CMB) datasets typically requires some filtering of the raw time-ordered data. Filtering is frequently used to minimize the impact of low frequency noise, atmospheric contributions and/or scan synchronous signals on the resulting maps. In this work we explicitly construct a general filtering operator, which can unambiguously remove any set of unwanted modes in the data, and then amend the map-making procedure in order to incorporate and correct for it. We show that such an approach is mathematically equivalent to the solution of a problem in which the sky signal and unwanted modes are estimated simultaneously and the latter are marginalized over. We investigate the conditions under which this amended map-making procedure can render an unbiased estimate of the sky signal in realistic circumstances. We then study the effects of time-domain filtering on the noise correlation structure in the map domain, as well as impact it may have on the performance of the popular pseudo-spectrum estimators. We conclude that although maps produced by the proposed estimators arguably provide the most faithful representation of the sky possible given the data, they may not straightforwardly lead to the best constraints on the power spectra of the underlying sky signal and special care may need to be taken to ensure this is the case. By contrast, simplified map-makers which do not explicitly correct for time-domain filtering, but leave it to subsequent steps in the data analysis, may perform equally well and be easier and faster to implement. We focus on polarization-sensitive measurements targeting the B-mode component of the CMB signal and apply the proposed methods to realistic simulations based on characteristics of an actual CMB polarization experiment, POLARBEAR.