Neutron Star Structure and Fundamental Physics

Neutron stars provide a natural laboratory for the study of matter under the most extreme conditions of density, pressure and magnetic fields known. Precise measurements of neutron star masses and radii may provide one of the only means to constrain important aspects of dense matter physics as well as fundamental particle physics. Moreover, neutron stars may contain states of matter not present anywhere else in the observable universe, for example, an exotic color-flavor-locked (CFL), super-fluid quark matter phase. Here, I discuss and review recent efforts to measure neutron star masses and radii in order to use them as probes of fundamental physics. Such efforts have focused on measurements of X-ray pulsation signals from both isolated and accreting neutron stars, as well as observations of the thermal emission from neutron star surfaces. Indeed, recent X-ray observations have provided the first evidence of discrete, redshifted spectral features from neutron star surfaces, which may eventually provide precise mass and radii estimates, however, uncertainties remain in their interpretation. Fast X-ray timing observations of X-ray bursting neutron stars have revealed spin modulation pulsations, and recent studies of “magnetar” giant flares have found fast oscillations which may be signatures of global vibrations of neutron star crusts. I will describe how these observations are yielding new insights on neutron star structure.