Determining neutron star masses and radii using energy-resolved waveforms of X-ray burst oscillations

Abstract

Simultaneous, precise measurements of the mass $M$ and radius $R$ of neutron stars can yield uniquely valuable information about the still uncertain properties of cold matter at several times the density of nuclear matter. One method that could be used to measure $M$ and $R$ is to analyze the energy-dependent waveforms of the X-ray flux oscillations seen during some thermonuclear bursts from some neutron stars. These oscillations are thought to be produced by X-ray emission from hotter regions on the surface of the star that are rotating at or near the spin frequency of the star. Here we explore how well $M$ and $R$ could be determined by generating, and analyzing using Bayesian techniques, synthetic energy-resolved X-ray data that we produce assuming a future space mission having 2--30 keV energy coverage and an effective area of 10 m$^2$, such as the proposed \textit{LOFT} or \textit{AXTAR} missions. We find that if the hot spot is within 10$^\circ$ of the rotation equator, both $M$ and $R$ can usually be determined with an uncertainty of about 10% if there are $10^6$ total counts from the spot, whereas waveforms from spots within 20$^\circ$ of the rotation pole provide no useful constraints. These constraints can usually be achieved even if the burst oscillations vary with time and data from multiple bursts must be used to obtain 10$^6$ counts from the hot spot. This is therefore a promising method to constrain $M$ and $R$ tightly enough to discriminate strongly between competing models of cold, high-density matter.