The Neutron-Star Equation of State and Gravitational Waves from Compact Binaries

Abstract

The equation of state (EOS) of matter above nuclear density is currently uncertain by almost an order of magnitude. Fortunately, neutron stars (NS) provide an ideal laboratory for studying high density matter. In order to systematize the study of the EOS from NS observations, we introduce a parametrized high-density EOS that accurately fits theoretical candidate EOSs. We then determine the ability of several recent and near-future electromagnetic observations to constrain the parameter space of our EOS. Recent observations include measurements of masses, gravitational redshift, and spin period, and we find that high mass observations are the most useful at constraining the EOS. Reliable simultaneous mass--radius measurements or mass--moment of inertia measurements in the near future, on the other hand, would provide a dramatically stronger constraint by requiring the allowed parameters to lie on a hypersurface of the full parameter space. In addition to electromagnetic observations, binary neutron star (BNS) and black hole-neutron star (BHNS) coalescence events observed with gravitational-wave detectors offer the potential to dramatically improve our understanding of the EOS. Information about the EOS is encoded in the waveform through tidal interactions, and for BNS systems, the inspiral waveform depends on the EOS through a single parameter called the tidal deformability. Using recent numerical BHNS simulations we find that the entire BHNS inspiral-merger-ringdown waveform also depends on the EOS exclusively through the same tidal deformability parameter. Using these BNS and BHNS waveforms, we examine the ability of second generation detectors now in construction and planned third generation detectors to extract information about the EOS.