The mass and radius of the neutron star in the bulge low-mass x-ray binary KS1731-260

Feryal Özel, Andrew Gould, Tolga Güver

Research output: Contribution to journalArticlepeer-review

51 Scopus citations


Measurements of neutron star masses and radii are instrumental in determining the equation of state of their interiors, understanding the dividing line between neutron stars and black holes, and 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 KS1731-260. 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 KS1731-260 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 approximately flat between 7 and 9kpc. Combining these measurements, we place a strong upper bound on the radius of the neutron star, R ≤ 12.5km, while confining its mass to M ≤ 2.1 M .

Original languageEnglish (US)
Article number5
JournalAstrophysical Journal
Issue number1
StatePublished - Mar 20 2012


  • X-rays: binaries
  • X-rays: individual (KS 1731?260)
  • stars: distances
  • stars: neutron

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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