Image credit: NASA's Goddard Space Flight Center/S. Wiessinger
Neutron stars (NS) are the most compact objects in the universe without an event horizon. Their observational signatures, composition and evolution relates to extreme states of matter, in that one deals with interparticle spacings of the order of femtometers - the length scale of atomic nuclei - and temperatures ranging from 106 to 1011 K. The immediate consequence is that, in contrast to ordinary materials whose thermodynamic properties are governed by the electromagnetic force, all four fundamental forces are involved in a major way. Their interplay leads to rich and complex phenomena not found under terrestrial conditions.
For the bulk properties of NS, such as their masses and radii, the equation of state (EoS) is decisive. It encompassed aspects of the microphysics of nucleonic matter and possibly a phase of quasi-free quarks at very high densities. The EoS is also crucial for astrophysical scenarios of binary systems in the context of NS/NS and NS/Black Hole (BH) merger events. These are considered to be prime sources of gravitational waves (GW) and the likely engines of short gamma-ray bursts. The EoS affects the merger dynamics, BH formation timescales, the precise form of gravitational wave and neutrino signals, any associated mass loss and r-process nucleosynthesis, as well as the attendant gamma-ray bursts and optical flashes. The cold EOS probed with NS, and the way it joins up with the hot EOS that determines explosive conditions, are also vital for the understanding the late stages of core collapse supernovae, including their gravitational wave signals.