Gravitational waves from neutron star mergers and their relation to the nuclear equation of state (1907.08534v3)

Abstract: In this article, I introduce ideas and techniques to extract information about the equation of state of matter at very high densities from gravitational waves emitted before, during and after the merger of binary neutron stars. I also review current work and results on the actual use of the first gravitational-wave observation of a neutron-star merger to set constraints on properties of such equation of state. In passing, I also touch on the possibility that what we observe in gravitational waves are not neutron stars, but something more exotic. In order to make this article more accessible, I also review the dynamics and gravitational-wave emission of neutron-star mergers in general, with focus on numerical simulations and on which representations of the equation of state are used for studies on binary systems.

• The paper demonstrates how gravitational wave signals, including tidal deformability and post-merger frequencies, can constrain the nuclear equation of state.
• The analysis of GW170817 highlights that inspiral tidal effects and remnant oscillations provide measurable indicators of neutron star radii and compactness.
• The study emphasizes that integrating gravitational and electromagnetic observations will refine nuclear physics models and probe phase transitions in extreme matter.

An Overview of Gravitational Waves from Neutron Star Mergers and their Implications for the Nuclear Equation of State

The paper of gravitational waves (GWs) from neutron star (NS) mergers provides a unique avenue for probing the properties of matter at supranuclear densities, specifically through insights into the nuclear equation of state (EoS). In this paper, the author delineates methods for extracting such information from the GW signals emitted during the pre-merger, merger, and post-merger phases of binary neutron star (BNS) systems. The article further explores current research progress, results derived from GW170817—the first observed NS merger—and introduces potential future directions for utilizing GW observations to refine constraints on the EoS.

Extracting EoS Information from Observations

The EoS describes how matter behaves at extremely high densities, a condition naturally achieved in the cores of NSs. GW observations offer a novel observational tool to explore this regime. There are primarily two stages in BNS mergers that provide information about the EoS: the inspiral phase, where tidal interactions between the NSs imprint signatures on the GW waveform, and the post-merger phase, where the dynamics and oscillations of the merger remnant offer gravitational signals sensitive to the star’s internal structure.

Inspiral Phase: During the inspiral, the tidal deformability parameter $\Lambda$, associated with the tidal interactions, becomes relevant. This dimensionless quantity is strongly dependent on the EoS, providing constraints on NS compactness, radii, and ultimately the pressure-density relationship of NS matter. The analysis of GW170817 has already yielded significant insights, suggesting relatively soft EoSs for NSs and ruling out excessively stiff models.

Post-Merger Phase: The dynamics after the merger, if the remnant does not promptly collapse to a black hole, are governed by the EoS. The GW spectrum in this phase exhibits characteristic frequencies that correlate with the NS radius or tidal deformability, potentially allowing for precise measurements of these quantities given future gravitational wave observations with improved sensitivity.

Implications for Nuclear Physics

From a theoretical standpoint, NS mergers and the associated GW signals present a strategic tool for exploring the phase diagram of nuclear matter. Potentially yielding insights into phase transitions from hadronic matter to deconfined quark matter, these transitions, often leading to the formation of hybrid stars, remain a key area of research. The presence of such transitions could manifest as changes in the post-merger GW signal or even influence the maximum mass of stable NSs.

Further constraints on the EoS, particularly at densities not accessible by terrestrial experiments, have significant implications for nuclear physics, specifically enhancing our understanding of nucleonic interactions and the roles of additional particles, such as hyperons, in dense matter.

Future Directions

The advances in GW astronomy post-GW170817 and similar future detections hold promise for further refining the constraints on the EoS. The integration of GW observations with electromagnetic counterparts (kilonovae and GRBs) bolsters the scientific dividends from these phenomenal cosmic laboratories. The synergy between astrophysical observations and advances in nuclear theory carries potential to challenge and refine our understanding of the fundamental properties of dense matter.

In prospect, the successful application of GW observations to the paper of the EoS not only exemplifies the synergy between observational astrophysics and theoretical modeling but also showcases the transformative impact of multi-messenger astronomy in broadening our comprehension of the universe. Continued advancements in detector sensitivity and analysis techniques will facilitate more robust conclusions regarding the structure of neutron stars and the properties of matter under extreme conditions.