The fundamental properties of neutron stars provide a direct test of the equation of state of cold nuclear matter, a relationship between pressure and density that is determined by the physics of the strong interactions between the particles that constitute the star. The most straightforward method of determining these properties is by measuring the gravitational redshift of spectral lines produced in the neutron star photosphere1. The equation of state implies a mass–radius relation, while a measurement of the gravitational redshift at the surface of a neutron star provides a direct constraint on the mass-to-radius ratio. Here we report the discovery of significant absorption lines in the spectra of 28 bursts of the low-mass X-ray binary EXO0748-676. We identify the most significant features with the Fe xxvi and xxv n = 2–3 and O viii n = 1–2 transitions, all with a redshift of z = 0.35, identical within small uncertainties for the respective transitions. For an astrophysically plausible range of masses (M ≈ 1.3–2.0 solar masses; refs 2–5), this value is completely consistent with models of neutron stars composed of normal nuclear matter, while it excludes some models6,7 in which the neutron stars are made of more exotic matter.
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This work is based on observations obtained with the XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and the USA (NASA). We thank E. Behar for supplying us with results from his atomic-structure calculations of the He-like Fe ion, and M. Sako for the use of his absorption spectral code.
The authors declare that they have no competing financial interests.
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Cottam, J., Paerels, F. & Mendez, M. Gravitationally redshifted absorption lines in the X-ray burst spectra of a neutron star. Nature 420, 51–54 (2002). https://doi.org/10.1038/nature01159
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