Neutron star parameter estimation from a NICER perspective

We focus on statistical estimation of neutron star parameters, conditional on X-ray data acquired by contemporary and future space telescopes. In particular, we discuss Bayesian inference of: (i) parameters such as masses and radii, which define the spacetime exterior to neutron stars; and (ii) parameters defined by a model of the equation of state of interior dense matter. We tailor to X-ray spectral-timing (also termed pulse-profile or light-curve modelling) with telescopes such as NICER (NASA’s Neutron Star Interior Composition Explorer, installed on the International Space Station). We develop a prototype open-source software package, X-PSI, for neutron star X-ray Pulsation Simulation and Inference. This package is designed to interface native likelihood functionality with open-source posterior sampling software, for use on CPU-nodes of high-performance computing systems. We apply the X-PSI modelling framework to NICER observations of the rotation-powered millisecond pulsar J0030+0451; we jointly constrain its mass and radius, together with the spatial structure of hot surface regions whose X-ray emission signatures are rotationally modulated (or pulsed). The analysis is a proof of concept and acts as a pathfinder for ongoing and future analyses of NICER data. The rotation-powered pulsars targeted by NICER are identified as a logical baseline source-class for community-supported development of an open-source package for neutron star X-ray spectral-timing: specifically, a package that enables the definition and estimation of fundamental parameters pertaining to relativistic stellar structure.