Constraining Nuclear-Matter Equations of State by Gravitational Waves from Black Hole-Neutron Star Binaries

Abstract

Coalescence of binary compact objects such as binary neutron stars (NS-NS) and black hole-neutron star (BH-NS) binaries is the most promising source for ground-based laser-interferometric gravitational-wave detectors such as LIGO1) and VIRGO.2) These binaries are in general formed as a result of two supernovae. Then, the orbital separation decreases gradually due to gravitational radiation reaction (i.e., two objects are in an inspiral motion), and eventually, merger happens. In most of the inspiral phase, two compact objects are well approximated by two point masses because their radius is much smaller than the orbital separation. However, throughout the late inspiral to the merger phases, the orbital evolution and resulting gravitational waveform depend on their finite-size effects. In particular, in the merger phase, they depend on the structure of the neutron star (NS). Here, we focus in particular on gravitational waves from BH-NS binaries in the final phase. The final fate of BH-NS binaries is classified into the following two categories. For the case that black hole (BH) mass (MBH) is sufficiently large, the tidal force near the BH horizon (which is proportional to M−2 BH) is not strong enough to tidally disrupt the companion NS even in the innermost stable circular orbit (ISCO). In such cases, the NS behaves approximately as a point mass throughout the whole evolution, and thus, the finite-size information (such as radius and density profile of the NS) is not reflected in gravitational waves emitted. By contrast, for the case that the BH mass is low (typically the mass should be smaller than 3MNS where MNS is the NS mass), the companion NS may be tidally disrupted. The condition for the onset of tidal disruption is roughly written as