Mergers of Binary Neutron Stars: Linking Simulations with Multi-messenger Observations
Institute for Theoretical Physics, Goethe University FIAS – Frankfurt Institute for Advanced Studies
Local Project ID:
HPC Platform used:
SuperMUC and SuperMUC-NG
Two major events are responsible for what is considered the “golden age” of relativistic astrophysics. One is the detection of gravitational waves from merging neutron stars heralding the beginning of the multimessenger age. The other is the effort of the Event Horizon Telescope collaboration culminating in the first image of a black hole. Both events have been aided by simulations that require HPC. With project pr27ju, several studies could be conducted well alligned with these type of simulations expanding our knowledge about these important astrophysical events.
The investigation conducted within this project can be summarized in two categories. One is the simulation of accreting black holes, which aided the interpretation of the world-famous image of the EHT collaboration. The second focus was on binary neutron stars (BNS) studying so-far unexplored physics like the creation of quark-gluon plasma during the merger and how to identify it from gravitational waves (GW) or the impact of spin during the BNS’s inspiral. In what follows an overview of the main results of both categories is presented.
2.1 Simulating the first image of a black hole
In April 2019 the EHT collaboration published the first image of a black hole. As one of the principal investigators Prof. Luciano Rezzolla together with his team used simulations of accreting black holes in order to aid the interpretation of this image. The simulated system describes the flow of a hot plasma around a supermassive black hole like the one in the center of the galaxy of M87. The results of these simulations were used to develop a database of synthetic images that were then compared to the observation. This allows to test not only the accuracy, but also whether the correct theory of gravity. Since the simulations were performed assuming Einstein’s theory of general relativy to be the correct description, the synthetic images have to match the observed one, which indeed is the case[1, 2, 3]. An example of such a synthetic image is shown in Fig. 1.
Note: For related video material, please see section 6 at the bottom of this page!
2.2 Unexplored physics in binary neutron star mergers
While a lot could already be learnt from the first detection of GWs from a BNS merger [4, 5] in terms of how matter fundamentally behaves under extreme temperatures and densities [6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17], there is as much left to still be explored. One reason is that currently only the lowfrequency GW signal from the inspiral can be detected. However, it is expected that within the next five years also the high-frequency postmerger signal can be detected. Already preparing for these detections now, within this project could be clarified how to identify a phase transition from hadronic matter to quark-gluon plasma from such a signal. In  for the first time BNS simulations were performed that include a realistic description of the matter allowing for such a phase transition. It was shown that the so-called ringdown signal, i.e., the GW signal after the merger remnant has collapse could reveal the production of quark matter during the postmerger stage.
In  the aforementioned simulations have been analyzed from a more microphysical point of view connecting the two disciplines of astro- and nuclear physics. For the first time it was analyzed which thermodynamic conditions are met before and after the merger taking into account the possibility of a phase transition. Fig. 2 visualizes this the among nuclear physicists well-known QCD phase diagram.
Following these results,  explored a similar scenario yielding a stable merger remnant with a quarkmatter core. The simulations show how the Fourier transform of the GW signal exposes this quark core. Fig. 3 compares the “standard” signal of a purely hadronic merger (left panel) with that of a merger with a phase transition (right panel). The formation of quark-gluon plasma is then revealed simply by counting the peaks in the spectrum. While the former case only has one characteristic peak, the latter shows two peaks – one stemming from the hadronic stage of the merger and the second from the stage after the phase transition. This provides the clearest signature yet for telling the production of quark-gluon plasma in our Universe from future GW detections.
Besides the possible existence of quark-gluon plasma inside neutron stars, another piece of information that has so far not been explored is the impact of spin during the inspiral of two neutron stars. Typical simulations assume the two neutron stars to be non-rotating before merging. In , however, simulations with spinning binary components were performed. It was found that this has a significant impact on the matter that is ejected from the system prior to merger as can be seen from Fig. 4. This in turn, alters the expected electromagnetic signal, called kilonova, like the one observed for GW170817 .
The project has been realized using our existing and well-tested code infrastructure. This consists of the general-relativisic magneto-hydrodynamics (GRMHD) code BHAC , which uses octree-based AMR and evolves the GRMHD equations in a stationary spacetime. This code has been used for obtaining the results described in Sec. 2.1. A code comparison has been performed  involving BHAC and making use of this allocation.
The results in Sec. 2.2 have been obtained using WhiskyTHC [24, 25] and FIL. The latter has been updated in order to make use of high-order numerical methods .
Furthermore, this allocation was also used for the development of the new code FRAC , which together with a GRMHD code like BHAC or FIL allows for the inclusion of radiation transport in simulations of astrophysical phenomena. While FRAC already represents the current state-of-the-art in terms of radiative transport, a brand new method going beyond this state-of-the-art has been developed . This so-called Lattice Boltzmann (LB) method promises a more accurate treatment for including radiation in the form of photons or neutrinos in astrophysical simulations. First simulations of relativistic jets have been performed making use of pr27ju. The results of these simulations are shown in Fig. 5 and show that currently employed radiation codes (right-most panel) yield significantly different results when compared to the new LB-method (middle panel) due to the intrinsic limitations of the former.
 Event Horizon Telescope Collaboration, K. Akiyama, A. Alberdi, W. Alef, K. Asada, R. Azulay, A.-K. Baczko, D. Ball, M. Baloković, J. Barrett, et al., First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole, Astrophys. J. Lett., 875, L1 2019.
 Event Horizon Telescope Collaboration, K. Akiyama, A. Alberdi, W. Alef, K. Asada, R. Azulay,
A.-K. Baczko, D. Ball, M. Baloković, J. Barrett, et al., First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring, Astrophys. J. Lett., 875, L5 2019
 Event Horizon Telescope Collaboration, K. Akiyama, A. Alberdi, W. Alef, K. Asada, R. Azulay,
A.-K. Baczko, D. Ball, M. Baloković, J. Barrett, et al., First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole, Astrophys. J. Lett., 875, L6 2019.
 The LIGO Scientific Collaboration, the Virgo Collaboration, B. P. Abbott, R. Abbott, T. D. Abbott, F. Acernese, K. Ackley, C. aAdams, T. Adams, P. Addesso, R. X. Adhikari, V. B. Adya, and et al., Multi-messenger Observations of a Binary Neutron Star Merger, Astrophys. J. Lett.848, L12 2017.
 Benjamin P. Abbott et al., Gravitational Waves and Gamma-Rays from a Binary Neutron Star
Merger: GW170817 and GRB 170817A, Astrophys. J. Lett.848, L13, 2017.
 B. Margalit and B. D. Metzger, Constraining the Maximum Mass of Neutron Stars from Multimessenger Observations of GW170817, Astrophys. J. Lett., 850, L19 2017.
 A. Bauswein, O. Just, H.-T. Janka, and N. Stergioulas, Neutron-star Radius Constraints from
GW170817 and Future Detections, Astrophys. J. Lett., 850, L34 2017.
 L. Rezzolla, E. R. Most, and L. R. Weih, Using Gravitational-wave Observations and Quasiuniversal Relations to Constrain the Maximum Mass of Neutron Stars, Astrophys. J. Lett., 852, L25 2018.
 M. Ruiz, S. L. Shapiro, and A. Tsokaros, GW170817, general relativistic magnetohydrodynamic simulations, and the neutron star maximum mass, Phys. Rev. D97, 021501 2018.
 E. Annala, T. Gorda, A. Kurkela, and A. Vuorinen, Gravitational-Wave Constraints on the Neutron-Star-Matter Equation of State, Phys. Rev. Lett.120, 172703 2018.
 D. Radice, A. Perego, F. Zappa, and S. Bernuzzi, GW170817: Joint Constraint on the Neutron Star Equation of State from Multimessenger Observations, Astrophys. J. Lett., 852, L29 2018.
 E. R. Most, L. R. Weih, L. Rezzolla, and J. Schaffner-Bielich, New Constraints on Radii and Tidal Deformabilities of Neutron Stars from GW170817, Phys. Rev. Lett.120, 261103 2018.
 M.W. Coughlin, T. Dietrich, B. Margalit, and B. D. Metzger, Multi-messenger Bayesian parameter inference of a binary neutron-star merger, arXiv e-prints 2018.
 G. F. Burgio, A. Drago, G. Pagliara, H.-J. Schulze, and J.-B.Wei, Are Small Radii of Compact Stars Ruled out by GW170817/AT2017gfo?, Astrophys. J., 860, 139 2018.
 I. Tews, J. Margueron, and S. Reddy, Critical examination of constraints on the equation of state of dense matter obtained from GW170817, Physical Review C98, 045804 2018.
 Masaru Shibata, Enping Zhou, Kenta Kiuchi, and Sho Fujibayashi, Constraint on the maximum mass of neutron stars using GW170817 event, Phys. Rev. D100, 023015 2019.
 Sven Koeppel, Luke Bovard, and Luciano Rezzolla, A General-relativistic Determination of the Threshold Mass to Prompt Collapse in Binary Neutron Star Mergers, Astrophys. J. Lett.872, L16 2019.
 E. R. Most, L. J. Papenfort, V. Dexheimer, M. Hanauske, S. Schramm, H. Stöcker, and L. Rezzolla, Signatures of Quark-Hadron Phase Transitions in General-Relativistic Neutron-Star Mergers, Physical Review Letters122, 061101 2019.
 Elias R. Most, L. Jens Papenfort, Veronica Dexheimer, Matthias Hanauske, Horst Stoecker, and Luciano Rezzolla, On the deconfinement phase transition in neutron-star mergers, European Physical Journal A56, 59 2020.
 Lukas R. Weih, Matthias Hanauske, and Luciano Rezzolla, Postmerger Gravitational-Wave Signatures of Phase Transitions in Binary Mergers, Phys. Rev. Lett.124, 171103 2020.
 E. R. Most, L. J. Papenfort, A. Tsokaros, and L. Rezzolla, Impact of High Spins on the Ejection of Mass in GW170817, Astrophys. J.884, 40 2019.
 O. Porth, H. Olivares, Y. Mizuno, Z. Younsi, L. Rezzolla, M. Moscibrodzka, H. Falcke, and M. Kramer, The black hole accretion code, Computational Astrophysics and Cosmology, 4, 1 2017.
 Oliver Porth, Koushik Chatterjee, Ramesh Narayan, Charles F. Gammie, Yosuke Mizuno, Peter Anninos, and et al., The Event Horizon General Relativistic Magnetohydrodynamic Code Comparison Project, Astrophys. J. Supp.243, 26 2019.
 D. Radice, L. Rezzolla, and F. Galeazzi, Beyond second-order convergence in simulations of binary neutron stars in full general-relativity, Mon. Not. R. Astron. Soc. L., 437, L46–L50 2014.
 D. Radice, L. Rezzolla, and F. Galeazzi, High-order fully general-relativistic hydrodynamics: new approaches and tests, Class. Quantum Grav.31, 075012 2014.
 E. R. Most, L. Jens Papenfort, and L. Rezzolla, Beyond second-order convergence in simulations of magnetized binary neutron stars with realistic microphysics, Mon. Not. R. Astron. Soc.490, 3588–3600 2019.
 Lukas R. Weih, Hector Olivares, and Luciano Rezzolla, Two-moment scheme for generalrelativistic radiation hydrodynamics: a systematic description and new applications, Mon. Not. R. Astron. Soc.495, 2285–2304 2020.
 L. R. Weih, A. Gabbana, D. Simeoni, L. Rezzolla, S. Succi, and R. Tripiccione, Beyond moments: relativistic Lattice-Boltzmann methods for radiative transport in computational astrophysics, arXiv e-prints, p. arXiv:2007.05718 2020
For some of the publications listed in Sec. 4 the simulations have been visualized in a way accessible for the general public. A video of…
An accreting black hole with a visual explanation of how a picture like the one taken by the EHT
collaboration can be extracted from such a simulation can be found here.
A BNS merger with a phase transition to quark-gluon plasma after the merger that triggers a
collapse to a black hole can be found here.
A BNS merger with a phase transition to quark-gluon-plasma that leads to a stable star with a
massive quark core can be found here.
Prof. Dr. Luciano Rezzolla (PI)1, 2, Dr. Dr. Matthias Hanauske1, Dr. Roman Gold1, Dr. Antonios Nathanail1
1 Institute for Theoretical Physics, Goethe University, Frankfurt
2 FIAS – Frankfurt Institute for Advanced Studies
Prof. Dr. Luciano Rezzolla
Institute for Theoretical Physics, Goethe University
FIAS – Frankfurt Institute for Advanced Studies
Max-von-Laue-Str. 1, D- 60438 Frankfurt am Main (Germany)
e-mail: rezzolla [@] itp.uni-frankfurt.de
Local Project ID: pr27ju