Equation of state dependence of neutron star mergers

Informazioni relative al progetto

EOSDNSM

ID dell’accordo di sovvenzione: 331873

Progetto chiuso

Data di avvio 1 Agosto 2013

Data di completamento 31 Luglio 2015

Finanziato da

• Specific programme "People" implementing the Seventh Framework Programme of the European Community for research, technological development and demonstration activities (2007 to 2013)

Costo totale € 161 968,80

Contributo UE € 161 968,80

Coordinato da

ARISTOTELIO PANEPISTIMIO THESSALONIKIS
Greece

Final Report Summary - EOSDNSM (Equation of state dependence of neutron star mergers)

Summary description of the project objectives

The project “Equation of state dependence of neutron star mergers” deals with numerical simulations of merging neutron stars. The studies follow two main goals: a deeper understanding of the gravitational-wave emission of these events focusing on the postmerger phase and an investigation of the role of mergers for the production of heavy elements formed by the rapid neutron-capture process (r-process). In particular, we aim to clarify the role of the high-density matter equation of state, which is still largely unknown. By exploring the observational implications of different proposed equations of state we develop strategies for determining observationally the properties of high-density matter.
The main objectives in the context of gravitational waves are the identification of the main mechanisms generating the different features of the gravitational-wave spectrum of the postmerger phase, which requires a theoretical model of neutron-star merger remnants. We want to quantify the equation-of-state dependence of the postmerger gravitational-wave signal and to build up a ready-to-use pipeline for the detection and interpretation of gravitational waves from neutron-star mergers.

Exploring the formation of heavy elements by neutron-star mergers requires an analysis of the matter which becomes gravitationally unbound from the merger site. Apart from exploring the robustness of the r-process in the ejecta by nuclear network calculations, we are interested in how the high-density matter equation of state affects the conditions of the unbound material, for instance its mass and outflow velocities. This is not only a relevant question for the overall production of r-process elements by neutron-star mergers, but also for the properties of potentially observable electromagnetic counterparts powered by the nucleosynthetic processes in the outflowing material.
Description of the work performed

We performed a large set of neutron-star merger simulations with many different temperature-dependent equations of state and with systematically varied binary masses. The gravitational-wave spectra were analyzed and empirical relations for different features of the spectrum were derived. Also the collapse behavior of the merger remnants was determined as function of the equation of state. Models were prepared for future use in a linear eigenvalue code. By a thorough investigation of the hydrodynamical data, oscillation modes and dynamical features of the merger remnant were identified and linked to certain features of the gravitational-wave signal. The dependence of these features on the equation of state and on the binary setup was studied. Our simulated gravitational-wave signals were used in gravitational-wave data analysis studies to explore the detectability and expected detection rate. This work included also the use of newly developed hybrid gravitational waveforms.

A new grid-based hydrodynamical code was written, which is ready to be used for the simulation of merger remnants. Also equilibrium models of merger remnants can be evolved.

The properties of neutron-star merger ejecta from many different simulations was explored. In particular, the equation-of-state dependence of the ejecta masses and outflow velocities was quantified. Additionally, these results were used to estimated the properties of electromagnetic counterparts by means of simple formulas connecting the ejecta properties with the characteristics of the electromagnetic display. We provided our data for dedicated nuclear network calculations to compute the final abundance patterns of heavy elements in the ejecta.
For the investigation of the impact of neutrino effects on the ejecta we implemented a neutrino leakage scheme in the existing relativistic smooth particle hydrodynamics code and undertook a first parametric study.

[Apart from these activities which followed closely the work packages of the proposal, we performed additional work which was originally not part of the proposal but is related to its objectives. This includes a new estimate of merger rates, the combined consideration of dynamical and secular ejecta of mergers, a prediction for a new type of early and bright electromagnetic counterpart, and the exploration of the impact of different nuclear models on the final abundance pattern in merger ejecta.]

Description of main results

Our enlarged set of merger simulations has fully confirmed our previous finding of a tight relation between the dominant gravitational-wave oscillation frequency and neutron-star radii. Inferring neutron-star radii from gravitational-wave observations rests on the precision of this relation. The analysis of these calculations allowed us to deduce the collapse behavior of neutron-star merger remnants. Interestingly, the threshold binary mass for a prompt collapse to a black hole immediately after merging depends in a particular way on the equation of state. This dependence can be employed to constrain or even to measure the maximum mass of non-rotating neutron stars by multiple detections of the gravitational-wave signal of neutron-star merger events. By a detailed investigation of the hydrodynamical data from our simulations we identified three different mechanisms, which produce the most pronounced features (peaks) in the gravitational-wave spectrum. We arrived at an understanding which of the different mechanisms are particularly strong for a given binary system and equation of state. This lead to a unified classification scheme of the postmerger dynamics and gravitational-wave emission. We quantified for the individual mechanisms how the corresponding gravitational-wave peak in the spectrum depends on the equation of state.

We developed hybrid gravitational waveforms based on our numerical data which are combined with an analytic description of the late-time behavior. These hybrid waveforms and our purely numerical waveforms were employed in a gravitational-wave data analysis study. This revealed that the dominant oscillation frequency can be accurately measured and thus will provides a tight constraint on the properties of high-density matter by a future measurement. Very recently, we also worked out an analytic model of the postmerger gravitational-wave signal, which may be employed as template in future gravitational-wave searches and may boost the sensitivity of the searches compared to the detectability found in our first study.

Determining the ejecta properties in many different simulations showed that the amount of unbound matter is larger for soft equations of state. This equation of state dependence is also imprinted on the peak luminosity of the light curve of electromagnetic counterparts. In contrast, the abundance pattern of heavy elements synthesized in the ejecta is insensitive to the equation of state or the chosen binary system. A first parametric study, however, indicated that neutrino interactions might have a gentle impact on the abundance pattern depending sensitively on details of the model. Expected final results and their impact and use

The project finished and all project objectives have been achieved. The major results were published in peer-reviewed high-impact journals and presented at international conferences including an international workshop on the subject organized by the host and the fellow at the host institution. The theoretical understanding of the merger dynamics and of the corresponding gravitational-wave emission will be crucial for the interpretation of future detections and will also serve as a guidance for the further numerical exploration of neutron-star mergers. Within the project we proved that a measurement of the dominant postmerger gravitational-wave frequency is possible, which would have a tremendous impact on the understanding of high-density matter by providing an accurate measurement of neutron-star radii. In the same way a determination of the maximum mass of neutron stars will have an impact on high-density matter physics. Our theoretical understanding will also have an impact on data analysis strategies for future gravitational-wave signals.

Our data describing the properties of the ejecta of merger events was provided to many different groups for post-processing nuclear network calculations. By this our work has an impact on the understanding of the relevant nuclear reactions and the underlying theoretical nuclear models. Our finding of a clear equation-of-state dependence of the ejecta properties is relevant for chemical evolution models as well as for the interpretation of future observations of electromagnetic counterparts.

Apart from the scientific progress we disseminated our results also to the general public. By advancing the theoretical understanding of fundamental physics our work may have indirectly a significant impact.