Measuring Neutron Star Radii via Pulse Profile Modeling with NICER

TL;DR

This paper assesses how well NICER can measure neutron star radii by modeling pulse profiles, emphasizing the importance of background knowledge and demonstrating potential for precise constraints on neutron star properties.

Contribution

It provides analytic estimates of background requirements for NICER's pulse profile modeling to accurately measure neutron star radii and constrain their equations of state.

Findings

Background knowledge at a few percent level enables <10% neutron star compactness measurement.

More realistic emission and spin assumptions improve constraints.

Additional source information enhances measurement accuracy.

Abstract

The Neutron-star Interior Composition Explorer (NICER) is an X-ray astrophysics payload that will be placed on the International Space Station. Its primary science goal is to measure with high accuracy the pulse profiles that arise from the non-uniform thermal surface emission of rotation-powered pulsars. Modeling general relativistic effects on the profiles will lead to measuring the radii of these neutron stars and to constraining their equation of state. Achieving this goal will depend, among other things, on accurate knowledge of the source, sky, and instrument backgrounds. We use here simple analytic estimates to quantify the level at which these backgrounds need to be known in order for the upcoming measurements to provide significant constraints on the properties of neutron stars. We show that, even in the minimal-information scenario, knowledge of the background at a few percent level for a background-to-source countrate ratio of 0.2 allows for a measurement of the neutron star compactness to better than 10% uncertainty for most of the parameter space. These constraints improve further when more realistic assumptions are made about the neutron star emission and spin, and when additional information about the source itself, such as its mass or distance, are incorporated.