The measurement of the neutron star radius

Sebastien Guillot

June 03, 2013

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Abstract: A physical understanding of the behaviour of cold ultra-dense matter — at and above nuclear density — can only be achieved by the study of neutron stars, and the thermal emission from quiescent low-mass X-ray binaries inside globular clusters have proven very useful for that purpose. The recent 1.97+/-0.04 Msun measurement for the radio pulsar PSR 1614-2230 suggests that strange quark matter and hyperons/kaons condensate equations of states (EoS) are disfavoured, in favour of hadronic “normal matter” EoSs. Over much of the neutron star mass-radius parameter space, “normal matter” EoSs produce lines of quasi-constant radii (within the measurement uncertainties, of about 10%). We present a simultaneous spectral analysis of several globular cluster quiescent low-mass X-ray binaries where we require the radius to be the same among all neutron stars analyzed. The Markov-Chain Monte-Carlo method and the Bayesian approach developed in this analysis permits including uncertainties in the distance, in the hydrogen column density, and possible contributions to the spectra due to un-modelled spectrally hard components. Our results suggest a neutron star radius much smaller than previously reported, with a value Rns = 9.1 (+1.3)(-1.5) km, at 90% confidence, using conservative assumptions, which suggests that neutron start matter is best described by the softest “normal matter” equations of state.