Gravitational waves from neutron star mergers and their relation to the nuclear equation of state

doi:10.1016/j.ppnp.2019.103714

Progress in Particle and Nuclear Physics

Volume 109, November 2019, 103714

https://doi.org/10.1016/j.ppnp.2019.103714 Get rights and content

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.

Section snippets

Opening words

For decades hope has grown that measurements of gravitational waves (GWs), and related electromagnetic radiation, from mergers of binaries composed of compact stars¹

Extracting information on the equation of state from gravitational waves emitted before the merger

The dynamics of BNS systems around merger depend on the EoS of ultrahigh-density matter. Essentially two methods to link the observed GWs to the NS EoS have been studied. One method uses tidal deformations during the last orbits before merger [67], [68], [69], [70], [71], [140], [245], [334], [400], [401], [402], [403], [404], [405], [406], [407], [408], [409], [410]; tidal deformations have been measured for GW170817²⁰

Extracting information on the equation of state from gravitational waves emitted after the merger

Also GW signals from the merger and post-merger phases of GW170817 have of course been searched in the data from Advanced LIGO and Advanced Virgo, but no signal was found [220], [502]. In fact, the strain upper limits set by the detector were found to be about one order of magnitude above the numerical-relativity expectations for post-merger emission from a hypermassive NS at the distance of GW170817.

The first detection of a BNS post-merger signal is still to come, but simulations and

Combining analyses of waveforms emitted before and after the merger

Even if (as mentioned in Section 3.3), with current detectors, omitting the post-merger waveforms from the global analysis of the signal does not lead to significant loss of information or biased estimation of the source properties [408], it would be ideal to have waveform templates that consistently and exhaustively cover the inspiral, merger, and post-merger phases, so that one could perform matched-filter searches, as mentioned in Section 4.2. This is true especially for the shorter-duration

The merger of binaries of stars not made of ordinary matter

As mentioned in Section 2.5, there may exist compact objects similar in mass and size to NSs, but not made of ordinary matter, like boson stars or gravastars. A few recent works have proposed preliminary studies about whether current and future observations can distinguish between inspirals and mergers of NSs and boson stars [95], [97], [538], [539], [540], [541] or gravastars [315], [331], [542]. GW detectors may also be able to probe the structure of these different compact objects (if they

Constraints on stellar radius and equation-of-state parameters deduced from GW170817

In this Section, I discuss the numerous works and results that appeared after the GW observation of GW170817 [1] and tried to estimate physical quantities from it and, ultimately, the EoS. Before starting, however, let me note that some articles [29], [30], [31] claimed that actually GW170817 has not added new insights about the EoS, because the constraints it imposes are less stringent than those obtained from current knowledge in nuclear physics, with the possible exceptions of the estimate

Conclusion

Even if not all agree, most people in the community are saying that, just from the one detection of GWs from BNS mergers that has been analysed, it has already been possible to set new constraints on the EoS of very high-density matter. It may be not so useful to debate on whether such a statement is true or not, since the new observation run of the LIGO-Virgo Collaborations has already started to give us new datasets on BNS mergers, which will definitively tell us more about the EoS of compact

Acknowledgements

I would like to thank all the colleagues who gave me valuable suggestions on how to improve this review. Partial support has come from JSPS, Japan Grant-in-Aid for Scientific Research (C) No. T18K036220.

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ReviewGravitational waves from neutron star mergers and their relation to the nuclear equation of state

Abstract

Section snippets

Opening words

Extracting information on the equation of state from gravitational waves emitted before the merger

Extracting information on the equation of state from gravitational waves emitted after the merger

Combining analyses of waveforms emitted before and after the merger

The merger of binaries of stars not made of ordinary matter

Constraints on stellar radius and equation-of-state parameters deduced from GW170817

Conclusion

Acknowledgements

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Review
Gravitational waves from neutron star mergers and their relation to the nuclear equation of state