Probing dense matter in compact star cores with radio pulsar data

Abstract

Astrophysical observations of compact stars provide, in addition to collider experiments, the other big source of information on matter under extreme conditions. The largest and most precise data set about neutron stars is the timing data of radio pulsars. We show how this unique data can be used to learn about the ultra-dense matter in the compact star interior. The method relies on astro-seismology based on special global oscillation modes (r-modes) that emit gravitational waves. They would prevent pulsars from spinning with their observed high frequencies, unless the damping of these modes, determined by the microscopic properties of matter, can prevent this. We show that for each form of matter there is a distinct region in a frequency/spindown-rate diagram where r-modes can be present. We find that stars containing ungapped quark matter are consistent with both the observed radio and x-ray data, whereas, even when taking into account the considerable uncertainties, neutron star models with standard viscous damping are inconsistent with both data sets and additional damping mechanisms would be required.