The Mass and Radius of the Neutron Star in the Bulge Low-Mass X-ray Binary KS 1731-260

TL;DR

This study measures the mass and radius of the neutron star in KS 1731-260 using spectroscopic data from thermonuclear bursts, providing constraints that inform the neutron star equation of state and its distinction from black holes.

Contribution

It presents the first well-constrained measurements of the neutron star's mass and radius in KS 1731-260, utilizing spectroscopic analysis combined with Galactic bulge distance estimation.

Findings

Neutron star radius R <= 12 km

Neutron star mass M <= 1.8 solar masses

Distance to source estimated at ~8 kpc

Abstract

Measurements of neutron star masses and radii are instrumental for determining the equation of state of their interiors, understanding the dividing line between neutron stars and black holes, and for obtaining accurate statistics of source populations in the Galaxy. We report here on the measurement of the mass and radius of the neutron star in the low-mass X-ray binary KS 1731-260. The analysis of the spectroscopic data on multiple thermonuclear bursts yields well-constrained values for the apparent angular area and the Eddington flux of the source, both of which depend in a distinct way on the mass and radius of the neutron star. The binary KS 1731-260 is in the direction of the Galactic bulge, allowing a distance estimate based on the density of stars in that direction. Making use of the Han & Gould model, we determine the probability distribution over the distance to the source, which is peaked at 8 kpc. Combining these measurements, we place a strong upper bound on the radius of the neutron star, R <= 12 km, while confining its mass to M <= 1.8 M_sun.