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

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Abstract

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 the Rossi 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.

Original languageEnglish (US)
Pages (from-to)268-272
Number of pages5
JournalMonthly Notices of the Royal Astronomical Society
Volume387
Issue number1
DOIs
StatePublished - Jun 2008

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neutron stars
bursts
radii
touchdown
x rays
M stars
expansion
X Ray Timing Explorer
photosphere
inclination
effect

Keywords

  • Equation of state
  • Methods: observational
  • Stars: neutron
  • X-rays: bursts

ASJC Scopus subject areas

  • Space and Planetary Science

Cite this

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title = "Biases for neutron star mass, radius and distance measurements from Eddington-limited X-ray bursts",
abstract = "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 the Rossi 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",
author = "Galloway, {D. K.} and Feryal Ozel and Dimitrios Psaltis",
year = "2008",
month = "6",
doi = "10.1111/j.1365-2966.2008.13219.x",
language = "English (US)",
volume = "387",
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journal = "Monthly Notices of the Royal Astronomical Society",
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TY - JOUR

T1 - Biases for neutron star mass, radius and distance measurements from Eddington-limited X-ray bursts

AU - Galloway, D. K.

AU - Ozel, Feryal

AU - Psaltis, Dimitrios

PY - 2008/6

Y1 - 2008/6

N2 - 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 the Rossi 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.

AB - 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 the Rossi 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.

KW - Equation of state

KW - Methods: observational

KW - Stars: neutron

KW - X-rays: bursts

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U2 - 10.1111/j.1365-2966.2008.13219.x

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JF - Monthly Notices of the Royal Astronomical Society

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