Abstract
Through a very careful analysis Kolopanis et al. (2022) 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 limit.
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