Neutron Star Masses and Radii from Quiescent Low-Mass X-ray Binaries. (arXiv:1305.3242v1 [astro-ph.HE])

Neutron Star Masses and Radii from Quiescent Low-Mass X-ray Binaries. (arXiv:1305.3242v1 [astro-ph.HE]):
A recent analysis (Guillot et al. 2013) of the thermal spectra of 5 quiescent
low-mass X-ray binaries in globular clusters, in which it was assumed that all
neutron stars have the same radius, determined the radius to be
R=9.1^{+1.3}_{-1.5} km to 90% confidence. However, the masses of the sources
were found to range from 0.86 solar masses to 2.4 solar masses and a
significant amount of the predicted M-R region violates causality and the
existence of a 2 solar mass neutron star. The study determined the amount of
Galactic absorption along the lines-of-sight from fitting the X-ray spectra and
assumed all sources possessed hydrogen atmospheres. We argue, from a Bayesian
analysis, that different interpretations of the data are strongly favored. Our
most-favored model assumes i) the equation of state of neutron star crusts is
well-understood, ii) the high-density equation of state is consistent with
causality and the existence of neutron stars at least as massive as 2 solar
masses, iii) that the Galactic absorption is determined either from the fits in
Guillot et al. (2013) or from independent HI surveys, and iv) that these
objects are well-described by either hydrogen or helium atmospheres. With these
assumptions, the 90% confidence radius range for 1.4 solar mass stars is 11.4
to 12.8 km, and the allowed range for radii of all neutron stars between 1.2
solar masses and 2.0 solar masses is 10.9 to 12.7 km. This result is in much
greater agreement with predictions of the equation of state from both nuclear
experiments and theoretical neutron matter studies than the smaller radii
deduced by Guillot et al. (2013).