Effects of Spot Size on Neutron-Star Radius Measurements from Pulse Profiles

TL;DR

This study assesses how the size of emission spots on neutron stars affects the accuracy of radius measurements from pulse profiles, finding that assuming small spots introduces less than 10% error for spots under 18 degrees.

Contribution

It quantifies the bias in neutron star radius measurements caused by assuming infinitesimally small spots, providing guidelines for when this approximation is valid.

Findings

Small spots (<10°-18°) cause ≤10% error in radius estimates.

The assumption of small spots is justified for certain source classes.

Implications for upcoming X-ray missions NICER and LOFT.

Abstract

We calculate the effects of spot size on pulse profiles of moderately rotating neutron stars. Specifically, we quantify the bias introduced in radius measurements from the common assumption that spots are infinitesimally small. We find that this assumption is reasonable for spots smaller than 10$^\circ$-18$^\circ$ and leads to errors that are $\le$10% in the radius measurement, depending on the location of the spot and the inclination of the observer. We consider the implications of our results for neutron star radius measurements with the upcoming and planned X-ray missions NICER and LOFT. We calculate the expected spot size for different classes of sources and investigate the circumstances under which the assumption of a small spot is justified.