Probing neutron star interiors with multi-messenger observations

One of the unsolved fundamental questions of modern astrophysics is the behaviour of matter at extreme densities and pressures, as found in the core of neutron stars. These conditions cannot be probed in the laboratory or be calculated from first principles, so progress has mostly come from observing astrophysical phenomena involving neutron stars. This thesis focuses on extracting information on the interior of neutron stars, governed by the equation of state (EoS), from these phenomena. The EoS relates the density to the pressure in the star and sets properties such as its mass, radius, and deformability. We develop a framework to infer the EoS from these properties (Ch. 2), which we then apply in Ch. 3 and Ch. 5 to recent pulsar mass-radius measurements from NASA’s Neutron Star Interior Composition Explorer (NICER), an X-ray timing telescope. We separately investigate the combined constraints when including information of the tidal deformability of neutron stars, as measured by LIGO/Virgo in the binary neutron star merger GW170817 (Ch. 4). Tidal deformability measurements of binary mergers can be improved through additional measurements of an accompanying electromagnetic signal. In Ch. 6 we develop a framework to jointly analyze the gravitational wave signal and the so-called kilonova, a UV-optical-infrared transient occurring in binary neutron star and some black hole – neutron star mergers. In Ch. 7 we improve this framework by using more sophisticated kilonova modeling and explore how UV telescopes can provide crucial information on the mechanism powering the early kilonova.