Measurement of the Radius of Neutron Stars with High S/N Quiescent Low-mass X-ray Binaries in Globular Clusters. (arXiv:1302.0023v1 [astro-ph.HE]):
This paper presents the measurement of the neutron star (NS) radius using the
thermal spectra from quiescent low-mass X-ray binaries (qLMXBs) inside globular
clusters (GCs). Recent observations of NSs have presented evidence that cold
ultra dense matter -- present in the core of NSs -- is best described by
"normal matter" equations of state (EoSs). Such EoSs predict that the radii of
NSs, Rns, are quasi-constant (within measurement errors, of ~10%) for
astrophysically relevant masses (Mns > 0.5 Msun). The present work adopts this
theoretical prediction as an assumption, and uses it to constrain a single Rns
value from five qLMXB targets with available high signal-to-noise X-ray
spectroscopic data. Employing a Markov-Chain Monte-Carlo approach, we produce
the marginalized posterior distribution for Rns, constrained to be the same
value for all five NSs in the sample. An effort was made to include all
quantifiable sources of uncertainty into the uncertainty of the quoted radius
measurement. These include the uncertainties in the distances to the GCs, the
uncertainties due to the Galactic absorption in the direction of the GCs, and
the possibility of a hard power-law spectral component for count excesses at
high photon energy, which are observed in some qLMXBs in the Galactic plane.
Using conservative assumptions,we found that the radius, common to the five
qLMXBs and constant for a wide range of masses, lies in the low range of
possible NS radii, Rns=9.1(+1.3)(-1.5) km (90%-confidence). Such a value is
consistent with low-res equations of state. We compare this result with
previous radius measurements of NSs from various analyses of different types of
systems. In addition, we compare the spectral analyses of individual qLMXBs to
previous works.
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