Modeling the merger of black holes with neutron stars and subsequent processes in one simulation
Numerical simulation of a black hole-neutron star merger. Density profiles are shown in blue and green, magnetic field lines penetrating black holes are shown in pink. Unbound material is shown in white its velocity with a green arrow. Credit: K. Hayashi (Kyoto University)
Using supercomputer calculations, scientists at the Max Planck Institute for Gravity Physics in Potsdam and from Japan showed a consistent picture for the first time: They modeled the complete process of a black hole colliding with a neutron star. In their study, they calculated the process from the final orbit through merging to the post-merger phase where, according to their calculations, high-energy gamma-ray bursts could occur. The results of their study have now been published in the journal D Physical Overview.
Nearly seven years have passed since the first detection of gravitational waves. On September 14, 2015, LIGO detectors in the US recorded the signal of two black holes merging from the depths of space. Since then, a total of 90 signals have been observed: from binary systems of two black holes or neutron stars, and also from mixed binaries. If at least one neutron star is involved in the merger, it is possible that not only gravitational wave detectors will observe the event, but also telescopes in the electromagnetic spectrum.
When two neutron stars merged in the event detected on August 17, 2017 (GW170817), about 70 telescopes on Earth and in space observed electromagnetic signals. In the two neutron star mergers with black holes observed so far (GW200105 and GW200115), no electromagnetic pairing of gravitational waves has been detected. But as more such events are measured with increasingly sensitive detectors, the researchers expect electromagnetic observations here too. During and after fusion, matter is removed from the system and electromagnetic radiation is generated. It may also produce short bursts of gamma rays, as observed by space telescopes.
For their study, the scientists selected two different model systems consisting of a spinning black hole and a neutron star. The mass of the black hole is set at 5.4 and 8.1 solar masses, and the mass of the neutron star is set at 1.35 solar masses. This parameter was chosen so that the neutron star is expected to be torn apart by tidal forces.
“We’re gaining insight into processes that last one to two seconds—sounds short, but actually a lot happened during that time: from the final orbit and perturbation of the neutron star by tidal forces, to the release of matter, to the formation of an accretion disk around a new black hole. birth, and further release of matter in the jet,” said Masaru Shibata, director of the Department of Computational Relativistic Astrophysics at the Max Planck Institute for Gravitational Physics in Potsdam. “This high-energy jet may also be the cause of the short gamma-ray burst, the origin of which is still a mystery. The simulation results also show that the ejected material must have synthesized heavy elements such as gold and platinum.”
What happened during and after the merger?
Simulations show that during the merging process the neutron star is torn apart by tidal forces. About 80% of the neutron star matter falls into the black hole within a few milliseconds, increasing its mass by about one solar mass. In the next 10 milliseconds or so, the neutron star material forms a one-armed spiral structure. Part of the material in the spiral arms is ejected from the system, while the rest (0.2-0.3 solar masses) forms an accretion disk around the black hole. When the accretion disk falls into the black hole after merging, it causes a focused jet-like stream of electromagnetic radiation, which can eventually produce a short burst of gamma rays.
Simulation lasts a few seconds
The “Sakura” departmental cluster computer took about 2 months to solve Einstein’s equations for a process that took about two seconds. “Such general relativistic simulations are very time consuming. That’s why research groups around the world have so far focused on short simulations,” explains Dr. Kenta Kiuchi, the group leader in Shibata’s department, developed the code. “In contrast, end-to-end simulations, as we are now doing for the first time, provide a consistent picture of the entire process for a binary initial state that is defined once at the start.”
Moreover, only with such long simulations were the researchers able to explore the mechanism for generating short gamma-ray bursts, which typically last one to two seconds.
Shibata and scientists in his department are already working on similar but even more complex numerical simulations to consistently model the collision of two neutron stars and the phase after merging.
Black holes and neutron stars merge unseen in dense star clusters
City of Hayashi et al, a general relativistic magnetohydrodynamic simulation of neutrino radiation of a few seconds of black hole-neutron star fusion, D Physical Overview (2022). DOI: 10.103/PhysRevD.106.023008
Provided by Max Planck Society
Quote: Modeling of the merger of a black hole with a neutron star and subsequent processes in one simulation (2022, 14 July) retrieved July 14, 2022 from https://phys.org/news/2022-07-merger-black-hole-neutron-star.html
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