Limits to Extracting Neutron-star Physics Constraints from NICER Pulse Profiles

Abstract Modeling energy-dependent X-ray pulse profiles from rotation-powered millisecond pulsars observed with Neutron Star Interior and Composition Explorer (NICER) has emerged as a promising avenue for measuring neutron-star radii and probing the equation of state of cold, ultradense matter. However, pulse profile models have often required an unwieldy number of parameters to account for complex surface emission geometries, introducing the risk of overfitting and degeneracies. To explore the number of model parameters that can be inferred uniquely, we perform a quantitative assessment of the information content in X-ray pulse profiles by applying Fourier methods. We determine the number of independent observables that can be reliably extracted from the pulse shapes, as well as from complementary X-ray spectral data obtained with XMM-Newton, for key NICER targets. Our analysis provides a framework for evaluating the match between model complexity and data constraints. It also demonstrates the importance of incorporating in the model the pulsed components of the magnetospheric nonthermal emission, which may often contribute significantly to the observed spectra. Our results highlight limitations in previous inferences of neutron-star radii from NICER observations, which may have incorporated model complexity not supported by the data.