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

Understand

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.