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

Inside a neutron star The neutron star EXO 0748–676 is unusual for a low-mass X-ray binary in that redshifted oxygen and iron spectral lines are present in light emitted from its surface. Matter in neutron stars is extremely dense, in a state that differs considerably from that found in the early Universe or in experimental systems on Earth, and astrophysical observation is the only way of probing this material. The multiple signals from EXO 0748–676 provide that probe. Data from the RXTE, EXOSAT and XMM-Newton satellites have been used to calculate the star's mass and radius, and the findings rule out all the ‘soft’ equations of state for neutron stars. A ‘soft’ equation at a given temperature and density is indicative of a lower pressure than given by a ‘stiff’ equation. If EXO 0748-676 is typical, exotic condensates and unconfined quarks can be can be excluded from centre of neutron stars. Instead, relatively conventional equations of state apply, equivalent to a regime of neutrons and protons.