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