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A two-solar-mass neutron star measured using Shapiro delay


Neutron stars are composed of the densest form of matter known to exist in our Universe, the composition and properties of which are still theoretically uncertain. Measurements of the masses or radii of these objects can strongly constrain the neutron star matter equation of state and rule out theoretical models of their composition1,2. The observed range of neutron star masses, however, has hitherto been too narrow to rule out many predictions of ‘exotic’ non-nucleonic components3,4,5,6. The Shapiro delay is a general-relativistic increase in light travel time through the curved space-time near a massive body7. For highly inclined (nearly edge-on) binary millisecond radio pulsar systems, this effect allows us to infer the masses of both the neutron star and its binary companion to high precision8,9. Here we present radio timing observations of the binary millisecond pulsar J1614-223010,11 that show a strong Shapiro delay signature. We calculate the pulsar mass to be (1.97 ± 0.04)M, which rules out almost all currently proposed2,3,4,5 hyperon or boson condensate equations of state (M, solar mass). Quark matter can support a star this massive only if the quarks are strongly interacting and are therefore not ‘free’ quarks12.

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Figure 1: Shapiro delay measurement for PSR J1614-2230.
Figure 2: Results of the MCMC error analysis.
Figure 3: Neutron star mass–radius diagram.


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P.B.D. is a Jansky Fellow of the National Radio Astronomy Observatory. J.W.T.H. is a Veni Fellow of The Netherlands Organisation for Scientific Research. We thank J. Lattimer for providing the EOS data plotted in Fig. 3, and P. Freire, F. Özel and D. Psaltis for discussions. The National Radio Astronomy Observatory is a facility of the US National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

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All authors contributed to collecting data, discussed the results and edited the manuscript. In addition, P.B.D. developed the MCMC code, reduced and analysed data, and wrote the manuscript. T.P. wrote the observing proposal and created Fig. 3. J.W.T.H. originally discovered the pulsar. M.S.E.R. initiated the survey that found the pulsar. S.M.R. initiated the high-precision timing proposal.

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Correspondence to P. B. Demorest.

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Demorest, P., Pennucci, T., Ransom, S. et al. A two-solar-mass neutron star measured using Shapiro delay. Nature 467, 1081–1083 (2010).

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