General Relativistic Moving-mesh Simulations of Neutron-Star Mergers

The ERC Relativistic Astrophysics Group at the GSI Helmholtz Centre in Darmstadt focuses on hydrodynamical computer simulations of neutron-star mergers (see movie). The main goal of the group is to link obseravbles of neutron-star mergers to fundamental physics questions. This concerns for instance the properties of high-density nuclear matter and the transition to a quark-gluon plasma at finite chemical potential, which may be extracted from gravitational-wave signals of neutron-star mergers. The team also works on the formation of heavy elements through the rapid neutron-capture process in the ejecta of neutron-star mergers. These nucleosynthetic processes power the emission of kilonovae - the electromagnetic counterparts of colliding neutron stars. The team develops novel numerical methods to extract information about the underlying mass ejection and element formation from observations. The work of the group is supported by an ERC Starting Grant (grant agreement No. 759253). The team is lead by Priv.-Doz. Dr. Andreas Bauswein.

In October 2022 we received an ERC synergy grant together with colleagues in Copenhagen (D. Watson), Belfast (S. Sim) and Dublin (P. Dunne), which allows us to continue our research within a broader scope.

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Research Highlight: Spherical kilonova explosion

The kilonova AT2017gfo associated with GW170817, the first neutron star merger detected by gravitational waves, has been a treasure trove for Nuclear Astrophysics. A kilonova emerges from the electromagnetic emission of the hot explosion of the material that is escaping the merger site. In these outflows heavy elements are formed and the nucleosynthesis process creating these elements heats the matter producing the electromagnetic emission. In a recent study, which was lead by colleagues from the Niels Bohr Institute, we investigated the geometry of the explosion. It came as a big surprise that the outflow is very spherical. This is a priori not obvious considering the large amount of angular momentum in the system, which stems from the orbital motion of the neutron stars. Hence, naively one would expect that the outflow features a pronounced symmetry axis instead of having the geomerty of a sphere. This observation is not only helpful to better understand kilonovae, but it may also be used as a new probe for cosmology to determine the Hubble constant. The analysis recently appeared in Nature. (See also the press release and our popular article in Sterne und Weltraum.)

Research Highlight: Dark Matter in Neutron Stars

Specific models of dark matter predict that it may accummulate in the gravitational potential neutron stars and form dark compact objects by self-interactions among the dark mMatter particles. The dark matter may be dragged along with the stars during a neutron star collision. We have simulated such a combined system of ordinary matter and dark matter (Physical Review D 107, 083002 (2023)). During a neutron star merger potential dark matter components remain bound to the system and reside in the central remnant without interacting with the baryonic matter other than gravitationally. As a result the dark matter components orbit in the central gravitational potential of the neutron star remnant and generate a long-lasting gravitational wave signal. Being fundamentally different from the signal produced by the baryonic matter, its specific signature can reveal the presence of dark matter and thus help to comprehend the properties of the still mysterious dark matter component of the Universe.

Research Highlight: Black-hole torus evolution

Material ejected into the interstellar medium by a neutron-star merger can be extremely massive and neutron rich and could therefore represent a major site of the rapid neutron-capture (r-) process that is responsible for the creation of many heavy elements, such as Gold and Uranium. Whether or not neutron-star mergers are a significant, or even dominant, source of r-process elements depends sensitively on the mass and level of neutron richness of the material released during and after the merger. A new study now investigated the conditions to achieve a high neutron density in outflows from a black-hole disk system, which is often left behind after a neutron-star merger. The study systematically clarifies the roles of individual weak interactions between neutrons, protons, electrons, and neutrinos for creating neutron-rich conditions. It is found that disks reach the highest neutron densities if they are massive enough for electrons to be degenerate, but not so massive for neutrino absorption to become dominant. These conditions happen to be realized exactly for disk masses in the range produced after typical neutron-star mergers. This provides strong support to the idea that neutron-star mergers are major r-process sites. (See also the press release).

Research Highlight: Black-hole formation in Neutron Star Mergers

Neutron stars are the densest stellar objects in the Universe. They cannot be arbitrarily massive because beyond some threshold the gravitational attraction becomes too strong and the gravitational collapse to a black hole is unavoidable. Therefore, the collision of two stable neutron stars can lead to the formation of a black hole. The outcome basically depends on the total mass of the system and whether or not it exceeds the threshold for black-hole formation. In a recent study, which appeared in Physical Review Letters, we investigated the outcome of neutron star mergers and under which conditions they result in a black hole. Interestingly, the exact threshold for the gravitational collapse depends sensitively on the properties of nuclear matter. In turn, these dependencies imply that the outcome of neutron star mergers can inform about still unknown properties of high-density matter. (See also the press release)

Research Highlight: Identifying a First-Order Phase Transition in Neutron-Star Mergers through Gravitational Waves

The transition from ordinary nucleonic matter to deconfined quark matter at higher densities is still mysterious. Possibly neutron stars feature a core of quark matter in their center. This offers the opportunity to learn about the hadron-quark pahse transition from observations of neutron stars. Based on computer simulations we identified an unambiguous signature of the presence of quark matter in the gravitational-wave signal of neutron-star mergers. The appearance of quark matter can be detected by comparing the signal of the premerger stage with the characteristic frequencies inferred from the postmerger phase. A significant increase of the postmerger gravitational-wave frequencies would be indicative of the occurrence of deconfied quark matter. The results of the study have been published in Physical Review Letters (see also the press release).