An Investigation of Systematic Effects from Background Priors on PSR J0740+6620 Radius Estimates Using Synthetic NICER and XMM-Newton Data

Abstract Accurate and precise measurements of neutron star radii provide invaluable information about the cold, dense matter in neutron star cores. Analyses of synthetic X-ray pulse waveform data for nonaccreting neutron stars of the type collected by the Neutron Star Interior Composition Explorer (NICER) have indicated that the mass and radius estimates extracted therefrom are robust against potential sources of systematic error, such as errors in the assumed pattern of the thermal X-ray emission from the surface of these stars. A potentially important but unexplored source of systematic error is misparameterization of unmodulated background components, which might bias the inferred radius, particularly when data from different telescopes are analyzed. In this study, we investigate the effects of the background model on radius estimates by jointly analyzing synthetic NICER and XMM-Newton data, using the ∼2.1 M ⊙ pulsar PSR J0740+6620 as a prototypical example. Our analysis shows that even if the background assumed in the model underestimates the actual background by a factor of more than 5, the resulting shift of the radius posterior from the true value of the radius corresponds to only ∼1 σ . In all the cases we examined, the Bayesian evidence for the correct background model is greater than for the incorrect background model. These results add to the evidence that analyses of NICER-like data provide accurate measurements of radii when the statistical sampling is thorough and the model fits the data well.