Differential Polarization Calibration: A Consistency Test for Cosmic Birefringence
Abstract
The cosmic microwave background birefringence angle β is exactly degenerate with a common instrumental polarization-angle offset. The Minami--Komatsu(MK) likelihood separates these quantities using Galactic foreground polarization, so its inferred detector angles merit a diagnostic that does not reuse the foreground-EB model. We develop a birefringence-blind differential estimator from antisymmetric and symmetric cross-spectra and apply it for the first time as an explicit consistency test of a foreground-assisted birefringence analysis. For a map pair, the common rotation cancels identically and the estimator measures Δαij=αi−αj; combining all pairs in a gauge-fixed network reconstructs the observable detector-dependent calibration pattern. Applied to eight Planck NPIPE detector-set maps, the differential reconstruction agrees with the MK pattern, giving χ2=8.18 for seven degrees of freedom (PTE=0.32), with all shared-data residuals below 1.8σ. Because both reconstructions use the same maps, this is an independent estimator and modeling route rather than an independent-data confirmation. The agreement therefore strengthens confidence in the calibration foundation of the MK-inferred β without providing an independent absolute measurement of it. A conditional common-mode reconstruction gives β=0.37±0.12∘. The differential test uses existing data and can serve as a validation layer for future EB-based birefringence analyses.
Versions
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BibTeX
@misc{lonappan2026differential,
title = {{Differential Polarization Calibration: A Consistency Test for Cosmic Birefringence}},
author = {Anto I. Lonappan and Brian Keating and Kam Arnold},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.09149},
eprint = {2609.09149},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.09149},
note = {CC0},
}
RIS
TY - UNPB
TI - Differential Polarization Calibration: A Consistency Test for Cosmic Birefringence
AU - Anto I. Lonappan
AU - Brian Keating
AU - Kam Arnold
PY - 2026
DA - 2026/09/08
PB - arXiv
DO - 10.48550/arxiv.2609.09149
UR - https://arxiv.org/abs/2609.09149
AB - The cosmic microwave background birefringence angle β is exactly degenerate with a common instrumental polarization-angle offset. The Minami--Komatsu(MK) likelihood separates these quantities using Galactic foreground polarization, so its inferred detector angles merit a diagnostic that does not reuse the foreground-EB model. We develop a birefringence-blind differential estimator from antisymmetric and symmetric cross-spectra and apply it for the first time as an explicit consistency test of a foreground-assisted birefringence analysis. For a map pair, the common rotation cancels identically and the estimator measures Δα_ij=α_i−α_j; combining all pairs in a gauge-fixed network reconstructs the observable detector-dependent calibration pattern. Applied to eight Planck NPIPE detector-set maps, the differential reconstruction agrees with the MK pattern, giving χ^2=8.18 for seven degrees of freedom (PTE=0.32), with all shared-data residuals below 1.8σ. Because both reconstructions use the same maps, this is an independent estimator and modeling route rather than an independent-data confirmation. The agreement therefore strengthens confidence in the calibration foundation of the MK-inferred β without providing an independent absolute measurement of it. A conditional common-mode reconstruction gives β=0.37±0.12^∘. The differential test uses existing data and can serve as a validation layer for future EB-based birefringence analyses.
ER -