Comparative Analysis of Neutron Star Models: Investigating Systematic Effects on Radius Estimates

The densest matter in the observable universe resides within neutron star cores, which is of interest to various scientific fields such as supernova studies and nuclear physics. Understanding the composition of this matter is crucial, yet laboratory experiments cannot explore it. Therefore, observations of neutron stars are essential. Notably, the neutron star mass-radius relation primarily depends on the equation of state (EOS), which for a neutron star is pressure as a function of energy density (P(e)). Precise measurements of macroscopic observables such as mass, radius, and tidal deformability help constrain the EOS, with NASA’s Neutron Star Interior Composition Explorer (NICER) mission offering promising X-ray data. NICER’s high-timing resolution observations of millisecond pulsars provide an extra dimension of data, potentially breaking degeneracies and reducing systematic errors. Our study focuses on various model comparisons to explain the emission from the neutron star surface. We run the nested sampler MultiNest to provide the initial conditions for an extensive analysis using the emcee Markov chain Monte Carlo sampler. These model comparisons include the shape of the hot spots, the atmospheric composition of the neutron star, and the distribution of temperatures within the hot spots. The actual pattern of spots on the neutron star’s surface is not of a simple shape or uniform temperature. However, in evaluating synthetic waveforms featuring simple shapes such as circles and ovals, we aim to identify potential biases in the radius when applying these different model assumptions.