What is the equation of state of matter in neutron stars?

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Neutron star cores harbor the highest density matter in the Universe. The properties of matter at such high densities is determined by the "strong force"—one of the four fundamental forces of nature. Neutron starts are astrophysical laboratories that give us the unique opportunity to probe the low-temperature, high-density regime of Quantum Chromodynamics (QCD), where a host of novel phenomena associated with QCD phase transitions may take place. More than three decades after the discovery of pulsars, there remains no consensus model for their emission across the electromagnetic spectrum. X-ray polarimetry and spectroscopy with IXO will provide key tests.

IXO will determine the radius and mass of several neutron stars to within several percent , providing strong constraints on the Neutron Star Equation of State. The neutron star surface will be probed via X-ray absorption spectroscopy: metals deposited at the surface via accretion provide spectroscopic features.

Additionally, at the very high magnetic fields found in magnetars (10¹⁴ Gauss), IXO will be able to observe quantum electrodynamics events via X-ray polarization. See IXO Technology: X-ray Polarimeter.

For more information, refer to IXO Astro 2010 Decadal White Papers: The Behavior of Matter Under Extreme Conditions, F. Paerels et al.