ASTROPHYSICS

The age of multi-messenger gravitational wave astronomy has arrived. The simultaneous detection of gravitational and electromagnetic waves from merging neutron stars has illustrated the importance of having high resolution numerical relativity simulations, performed on SuperMUC, available to disentangle the complex interplay of nuclear physics, neutrino physics, and strong field gravity. Using these simulations, it is possible to study matter at densities unreachable with terrestrial experiments and determine the origin of the heavy elements in the universe.

Introduction

It has been a great fortune for the underlying project that its main purpose, which was formulated in the prognosticated title of the project has been acknowledged. Not even two years after the first detection of a gravitational wave (GW) emanated from the inward spiral and merger of pairs of a black holes by LIGO (GW150914), GWs from a binary neutron star merger has been recently discovered. In August 2017 GWs and electromagnetic counterparts were detected from the merger of binary neutron stars by the LIGO/Virgo collaboration and numerous observatories around the world. This long-awaited event (GW170817) marks the beginning of the new field of multi-messenger gravitational wave astronomy. Exploiting the extracted tidal deformations of the two neutron stars from the late inspiral phase of GW170817 it is now possible to severely constrain several global properties of the equation of state (EOS) of dense matter. However, the most interesting part of the high density and temperature regime of the EOS is solely imprinted in the post-merger GW emission from the remnant hypermassive/supramassive neutron star (HMNS/SMNS). This regime was not observed in GW170817, but will possibly be detected in forthcoming events within the next observing run. Based on a large number of numerical-relativity simulations, the emitted GWs, the interior structure of the generated HMNS/SMNS, the accurate measurement of the amount of ejected material from the merger, the synthetic light curves of the produced kilonova signal, the distribution of the abundances of heavy-elements and last but not least, the impact of magnetic fields on the long term ejection of mass have been investigated in detail within the underlying project pr62do.

Results and Methods

A multiplicity of quasi-circular and parabolic binary neutron star simulations have been performed in pure general relativistic hydrodynamics. Three finite temperature EOSs, three initial masses and two mass ratios have been explored in the quasi-circular runs, while the different simulations of the parabolic encounters contain two finite temperature EOSs, two mass ratios and six different values of the impact parameter. Based on these simulations, the internal and rotational HMNS/SMNS properties, the evolution of the density and temperature profiles of the remnant HMNS/SMNS and their connection with the emitted GW signal have been analyzed in detail [1,2]. Additionally, the accurate measurement of “dynamical ejecta” from the merger of binary neutron stars have been investigated. The merger is an extremely disruptive process, especially if the stars do not have the same mass or do not merge from quasi-circular orbits but through a dynamical capture. Mass can be ejected either very rapidly -- via tidal torques at the time of the dynamically merger or encounter -- or more slowly -- via winds that can be due to a number of different processes, which range from shock-heating to neutrino emission. This gravitationally unbound matter represents the perfect site for r-process nucleosynthesis and, if containing sufficient mass, can also lead to a bright electromagnetic signal, known as a “kilonova”, as the material decays radioactively. In the follow-up observations of GW170817, a kilonova was observed providing the first definitive and undisputed confirmation of a kilonova and the formation of r-process elements from merging neutron stars. To investigate the r-process formation in merging neutron stars, a variety of simulations were performed [2] using numerous EOSs, initial masses, and mass ratios which well sample the parameter space of BNS mergers.

Results and Methods

A multiplicity of quasi-circular and parabolic binary neutron star simulations have been performed in pure general relativistic hydrodynamics. Three finite temperature EOSs, three initial masses and two mass ratios have been explored in the quasi-circular runs, while the different simulations of the parabolic encounters contain two finite temperature EOSs, two mass ratios and six different values of the impact parameter. Based on these simulations, the internal and rotational HMNS/SMNS properties, the evolution of the density and temperature profiles of the remnant HMNS/SMNS and their connection with the emitted GW signal have been analyzed in detail [1,2]. Additionally, the accurate measurement of “dynamical ejecta” from the merger of binary neutron stars have been investigated. The merger is an extremely disruptive process, especially if the stars do not have the same mass or do not merge from quasi-circular orbits but through a dynamical capture. Mass can be ejected either very rapidly -- via tidal torques at the time of the dynamically merger or encounter -- or more slowly -- via winds that can be due to a number of different processes, which range from shock-heating to neutrino emission. This gravitationally unbound matter represents the perfect site for r-process nucleosynthesis and, if containing sufficient mass, can also lead to a bright electromagnetic signal, known as a “kilonova”, as the material decays radioactively. In the follow-up observations of GW170817, a kilonova was observed providing the first definitive and undisputed confirmation of a kilonova and the formation of r-process elements from merging neutron stars. To investigate the r-process formation in merging neutron stars, a variety of simulations were performed [2] using numerous EOSs, initial masses, and mass ratios which well sample the parameter space of BNS mergers.

Figure 1: Heavy-elements abundances (filled circles) versus the mass number A when computed for different EOSs, masses and mass ratios (shown with different lines). The left, middle and right panels refer to the DD2, the LS220, and the SFHO EOS, respectively. The vertical lines mark a few representative r-process elements: Figure taken from [2].
Copyright: ITP, Goethe University Frankfurt

Measuring the ejected material from these simulations we found that the amount of ejected material is on the order of ~10-3 Msolar but sensitively depends on the numerical parameters, such as grid resolution, unbound criterion, and neutrino treatment. Using a novel tracer method [2], the fluid elements could be followed along fluid lines which allowed for an accurate computation of the results from r-process nucleosynthesis. The result of this nucleosynthesis is displayed in Fig.1 for all the simulations in [2]. These simulations demonstrate that the r-process elements created from mergers is “robust”, in that it that is almost entirely independent on the initial masses, mass ratios, or EOS. Additionally, tracer data was also used to compute kilonova light curves. When comparing the produced light curves from the different simulations with those observed, show that our results are significantly dimmer than those observed, which was due lower ejected amount of ejected material and a lack of lanthanides. This suggests that the dynamical ejecta is not the major source of ejecta from a merger, but places a secondary role to other forms of secular ejecta, such as from neutrino driven winds or viscous ejecta from a disk.

A further use of the data produced from the simulations of [2], was used to investigate the effects of viscous dissipation in the post-merger of BNS mergers [3]. It was found by analyzing the data that the viscous effect of bulk viscosity can play an important role in post-merger dynamics which can be measured through the gravitational wave. This implies that the assumption of a perfect fluid inside the HMNS/SMNS needs to be relaxed to allow for viscous effects. The implementation of the relevant viscous contributions is presently under construction.

Another more uncommon type of merging BNS systems are highly eccentric mergers. These systems can form in environments of high stellar density as globular clusters as opposed to the primordial systems which lead to quasi-circular mergers. We carried out a series of simulations including the same mass ratio and EOSs as in the quasi-circular models described above, as well as different orbital configurations to determine the amount and properties of the ejected material. Depending on the EOS and the mass ratio we showed that the outflow can reach almost 10^-1 M_solar, which is significantly more than in the quasi-circular models and suggest a clear alteration of a kilonova signal coming from such mergers. Despite the fact that the thermodynamic properties of the dynamical ejecta differ considerably between the different models, the resulting r-process nucleosynthesis leads to almost the same abundances patterns as the quasi-circular models emphasizing the “robustness” of this process. Additionally, the gravitational wave signals coming from merging eccentric binaries have been analyzed and a selection is depicted in Fig. 2. Depending on the impact parameter the system undergoes multiple close encounters, where in each of these part of the orbital energy and angular momentum are radiated away in a burst of GWs. Due to the strong tidal effects the stars start to oscillate and radiate GWs with their f-mode frequency until they merge eventually. Especially the burst signals could be observed with future GW detectors from which one could deduce the position of the following kilonova emission.