Exploring Waveform Variations among Neutron Star Ray-tracing Codes for Complex Emission Geometries

TL;DR

This paper compares different neutron star ray-tracing codes for complex hot spot geometries to ensure waveform accuracy in NICER pulse profile modeling, confirming high overall accuracy but noting potential issues in extreme cases.

Contribution

It extends previous code verification to complex hot spot geometries, enhancing confidence in waveform modeling for NICER data analysis.

Findings

Waveform accuracy is generally sufficient for NICER data analysis.

Some extreme hot spot configurations require careful handling.

The study improves reliability of neutron star parameter inference.

Abstract

Pulse Profile Modeling (PPM), the technique used to infer mass, radius and geometric parameters for rotation-powered millisecond pulsars using data from the Neutron Star Interior Composition Explorer (NICER), relies on relativistic ray-tracing of thermal X-ray photons from hot spots on the neutron star surface to the observer. To verify our ray-tracing codes we have in the past conducted cross-tests for simple hot spot geometries, focusing primarily on the implementation of the space-time model. In this paper, we present verification for test problems that explore the more complex hot spot geometries that are now being employed in the NICER PPM analyses. We conclude that the accuracy of our computed waveforms is in general sufficiently high for analyses of current NICER data sets. We have however identified some extreme configurations where extra care may be needed.