Prospects for Neutron Star Equation of State Constraints using "Recycled" Millisecond Pulsars

TL;DR

This paper reviews how detailed modeling of X-ray pulsations from recycled millisecond pulsars can help determine neutron star mass-radius relations, informing the understanding of dense matter physics.

Contribution

It summarizes current modeling techniques, past observational results, and prospects for future measurements with NICER to constrain neutron star equations of state.

Findings

Modeling of X-ray pulsations can reveal neutron star structure.

Past observations have provided initial constraints.

Future NICER data promises improved mass-radius measurements.

Abstract

Rotation-powered "recycled" millisecond pulsars are a variety of rapidly-spinning neutron stars that typically show thermal X-ray radiation due to the heated surface of their magnetic polar caps. Detailed numerical modeling of the rotation-induced thermal X-ray pulsations observed from recycled millisecond pulsars, including all relevant relativistic and stellar atmospheric effects, has been identified as a promising approach towards an astrophysical determination of the true neutron star mass-radius relation, and by extension the state of cold matter at densities exceeding those of atomic nuclei. Herein, I review the basic model and methodology commonly used to extract information regarding neutron star structure from the pulsed X-ray radiation observed from millisecond pulsars. I also summarize the results of past X-ray observations of these objects and the prospects for precision neutron star mass-radius measurements with the upcoming Neutron Star Interior Composition Explorer (NICER) X-ray timing mission.