A Counterintuitive Correlation between Neutron Star Radii Inferred from Pulse Modeling and Surface Emission Beaming

Thermal X-ray emission from rotation-powered millisecond pulsars, shaped by gravitational lensing and the beaming of the surface radiation, provides critical insights into neutron star properties. This approach has been the focus of observations with the Neutron Star Interior Composition Explorer (NICER) mission. Using a semianalytic model to calculate pulse profiles, we investigate the effects of adopting incorrect beaming models on the inferred compactness of neutron stars. We demonstrate that assuming a more centrally peaked beaming pattern when fitting data from a more isotropic emitter leads to an underestimation of compactness in the case of two antipodal polar caps. We present a detailed analysis of this counterintuitive result, offering both qualitative insights and quantitative estimates. If the atmospheric heating in the millisecond pulsars observed with NICER is shallow, the inferred radii for these sources could be significantly overestimated, with important implications for neutron star structure and equation-of-state constraints.