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Soft equations of state for neutron-star matter ruled out by EXO 0748 - 676


The interiors of neutron stars contain matter at very high densities, in a state that differs greatly from those found in the early Universe or achieved in terrestrial experiments1. Matter in these conditions can only be probed through astrophysical observations that measure the mass and radius of neutron stars with sufficient precision2. Here I report a determination of the mass and radius of the neutron star EXO 0748 - 676 that appears to rule out all the soft equations of state of neutron-star matter. If this object is typical, then condensates2 and unconfined quarks1 do not exist in the centres of neutron stars.

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Figure 1: Four complementary methods to determine the mass and radius of a neutron star.
Figure 2: The constraints on neutron star equations of state imposed by observations of EXO 0748 - 676.


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I am grateful to the γ-ray group, especially C. Kouveliotou, for their hospitality at MSFC where this idea was born; and for the hospitality of the members of Anton Pannekoek Institute at the University of Amsterdam, where the work was completed. I thank D. Psaltis for useful discussions and comments on the manuscript.

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Correspondence to F. Özel.

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Özel, F. Soft equations of state for neutron-star matter ruled out by EXO 0748 - 676. Nature 441, 1115–1117 (2006).

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