James M. Lattimer, Andrew W. Steiner
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).
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http://arxiv.org/abs/1305.3242
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