Journal Article

Biases for neutron star mass, radius and distance measurements from Eddington-limited X-ray bursts

D. K. Galloway, F. Özel and D. Psaltis

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 387, issue 1, pages 268-272
Published in print June 2008 | ISSN: 0035-8711
Published online May 2008 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.13219.x
Biases for neutron star mass, radius and distance measurements from Eddington-limited X-ray bursts

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Eddington-limited X-ray bursts from neutron stars can be used in conjunction with other spectroscopic observations to measure neutron star masses, radii and distances. In order to quantify some of the uncertainties in the determination of the Eddington limit, we analysed a large sample of photospheric radius-expansion thermonuclear bursts observed with theRossi X-ray Timing Explorer. We identified the instant at which the expanded photosphere ‘touches down’ back on to the surface of the neutron star and compared the corresponding touchdown flux to the peak flux of each burst. We found that for the majority of sources, the ratio of these fluxes is smaller than ≃1.6, which is the maximum value expected from the changing gravitational redshift during the radius expansion episodes (for a 2 M neutron star). The only sources for which this ratio is larger than ≃1.6 are high-inclination sources that include dippers and Cyg X-2. We discuss two possible geometric interpretations of this effect and show that the inferred masses and radii of neutron stars are not affected by this bias. On the other hand, systematic uncertainties as large as ∼50 per cent may be introduced to the distance determination.

Keywords: equation of state; methods: observational; stars: neutron; X-rays: bursts

Journal Article.  4218 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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