Scaling relations for the uncertainty in neutron star radius inferred from pulse profile modelling: the effect of spin rate

Pulse profile modelling using X-ray data from NICER (the Neutron Star Composition ExploreR) permits the inference of mass and radius for rotation-powered millisecond pulsars. This in turn constrains the equation of state of cold dense matter. Previous studies indicate that the uncertainty in the inferred radius should reduce as neutron star spin rate increases. Here we test this using one of the pipelines currently being used for pulse profile modelling with NICER data. We synthesize a set of pulse profiles, assuming different neutron star spin frequencies, spanning the range (25–700) Hz. All of the simulated data sets are generated with the same (single) hotspot configuration, assuming a neutron star mass and radius of 1.6 M_☉ and 10 km. For this restricted set of synthetic data, we find no improvement in the radius credible interval once spin frequency exceeds a certain value (in this specific case ∼200 Hz). If this result were to apply more generally, it would have important implications for the observing strategy for current and future pulse profile modelling missions: targets can be prioritized based on properties other than their spin frequencies, as long as we are in the millisecond range.