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

TL;DR

This study investigates how neutron star spin rate affects the uncertainty in radius measurements from pulse profile modelling, finding no significant improvement beyond certain spin frequencies in synthetic data simulations.

Contribution

It provides a systematic analysis of the impact of spin rate on radius inference uncertainty using synthetic pulse profiles, challenging previous assumptions.

Findings

No reduction in radius uncertainty beyond ~200 Hz spin rate in simulations

Spin frequency may not be the primary factor in improving radius measurements

Implications for target selection in pulse profile modelling missions

Abstract

Pulse profile modelling using X-ray data from NICER 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) hot spot configuration, assuming a neutron star mass and radius of $1.6\,M_{\mathrm{\odot}}$ 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 $\sim 200$ 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.