Article

# Systematic Uncertainties in the Spectroscopic Measurements of Neutron-Star Masses and Radii from Thermonuclear X-ray Bursts. I. Apparent Radii

(Impact Factor: 5.99). 03/2011; 747(1). DOI: 10.1088/0004-637X/747/1/76
Source: arXiv

ABSTRACT

The masses and radii of low-magnetic field neutron stars can be measured by
combining different observable quantities obtained from their X-ray spectra
during thermonuclear X-ray bursts. One of these quantities is the apparent
radius of each neutron star as inferred from the X-ray flux and spectral
temperature measured during the cooling tails of bursts, when the thermonuclear
flash is believed to have engulfed the entire star. In this paper, we analyze
13,095 X-ray spectra of 446 X-ray bursts observed from 12 sources in order to
assess possible systematic effects in the measurements of the apparent radii of
neutron stars. We first show that the vast majority of the observed X-ray
spectra are consistent with blackbody functions to within a few percent. We
find that most X-ray bursts follow a very well determined correlation between
X-ray flux and temperature, which is consistent with the whole neutron-star
surface emitting uniformly during the cooling tails. We develop a Bayesian
Gaussian mixture algorithm to measure the apparent radii of the neutron stars
in these sources, while detecting and excluding a small number of X-ray bursts
that show irregular cooling behavior. This algorithm also provides us with a
quantitative measure of the systematic uncertainties in the measurements. We
find that those errors in the spectroscopic determination of neutron-star radii
that are introduced by systematic effects in the cooling tails of X-ray bursts
are in the range $\simeq 3-8$%. Such small errors are adequate to distinguish
between different equations of state provided that uncertainties in the
distance to each source and the absolute calibration of X-ray detectors do not
dominate the error budget.

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##### Article: Systematic Uncertainties in the Spectroscopic Measurements of Neutron-Star Masses and Radii from Thermonuclear X-ray Bursts. II. Eddington Limit
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ABSTRACT: Time resolved X-ray spectroscopy of thermonuclear bursts observed from low mass X-ray binaries offer a unique tool to measure neutron star masses and radii. In this paper, we continue our systematic analysis of all the X-ray bursts observed with RXTE from X-ray binaries. We determine the events which show clear evidence for photospheric radius expansion and measure the Eddington limits for these accreting neutron stars using the bolometric fluxes attained at the touchdown moments of each X-ray burst. We employ a Bayesian technique to investigate the degree to which the Eddington limit for each source remains constant between bursts. We find that for sources with a large number of radius expansion bursts, systematic uncertainties are at a 5-10% level. Moreover, in six sources with only pairs of Eddington-limited bursts, the distribution of fluxes is consistent with a ~10% fractional dispersion. This indicates that the spectroscopic measurements of neutron star masses and radii using thermonuclear X-ray bursts can reach the level of accuracy required to distinguish between different neutron star equations of state, provided that uncertainties related to the overall flux calibration of X-ray detectors are of comparable magnitude.
Full-text · Article · Apr 2011 · The Astrophysical Journal
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##### Article: The Mass and Radius of the Neutron Star in the Bulge Low-Mass X-ray Binary KS 1731-260
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ABSTRACT: 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 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 approximately flat between 7 and 9 kpc. Combining these measurements, we place a strong upper bound on the radius of the neutron star, R ≤ 12.5 km, while confining its mass to M ≤ 2.1 M ☉.
Full-text · Article · Apr 2011 · The Astrophysical Journal
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##### Article: Holographic equations of state and astrophysical compact objects
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ABSTRACT: We solve the Tolman-Oppenheimer-Volkoff equation using an equation of state (EoS) calculated in holographic QCD. The aim is to use compact astrophysical objects like neutron stars as an indicator to test holographic equations of state. We first try an EoS from a dense D4/D8/\textoverline {D8} model. In this case, however, we could not find a stable compact star, a star satisfying pressure-zero condition with a radius $R$, $p(R)=0$, within a reasonable value of the radius. This means that the EoS from the D4/D8/\textoverline {D8} model may not support any stable compact stars or may support one whose radius is very large. This might be due to a deficit of attractive force from a scalar field or two-pion exchange in the D4/D8/\textoverline {D8} model. Then, we consider D4/D6 type models with different number of quark flavors, $N_f=1,2,3$. Though the mass and radius of a holographic star is larger than those of normal neutron stars, the D4/D6 type EoS renders a stable compact star.
Preview · Article · Aug 2011 · Journal of High Energy Physics