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

TL;DR

This paper assesses how much information about neutron star radii can be reliably extracted from NICER pulse profiles, emphasizing the need to balance model complexity with data constraints to avoid overfitting.

Contribution

It introduces a Fourier-based framework to quantify the information content in X-ray pulse profiles and spectral data for neutron star radius measurements.

Findings

Limited independent observables can be extracted from pulse profiles.

Inclusion of magnetospheric non-thermal emission is crucial.

Previous radius inferences may have overfit the data.

Abstract

Modeling energy-dependent X-ray pulse profiles from rotation-powered millisecond pulsars observed with NICER has emerged as a promising avenue for measuring neutron star radii and probing the equation of state of cold, ultra-dense 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 non-thermal 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.