The Statistics of Negative Power Spectrum Systematics in some 21 cm Analyses

TL;DR

This paper investigates the origin and characteristics of negative power spectrum systematics in 21 cm cosmology, revealing their dependence on analysis choices and baseline effects, and offers guidelines for reporting upper limits.

Contribution

It introduces a statistical model explaining how baseline-dependent systematics cause negative power spectra, aiding accurate interpretation of 21 cm observational data.

Findings

Negative PS systematics can arise from analysis choices and baseline effects.

A statistical model characterizes the behavior of negative PS systematics.

Recommendations are provided for reporting upper limits considering negative PS.

Abstract

Through a very careful analysis Kolopanis and collaborators identified a negative power spectrum (PS) systematic. The 21 cm cosmology community has assumed that any observational systematics would add power, as negative PS are non-physical. In addition to the mystery of their origin, negative PS systematics raise the spectre of artificially lowering upper limits on the 21 cm PS. It appears that the source of the negative PS systematics is a subtle interaction between choices in how the PS estimate is calculated and baseline-dependent systematic power. In this paper we present a statistical model of baseline dependent systematics to explore how negative PS systematics can appear and their statistical characteristics. This leads us to recommendations on when and how to consider negative PS systematics when reporting observational 21 cm cosmology upper limits.