Modeling the merger of a black hole with a neutron star and the subsequent process in a single simulation

Modeling the merger of a black hole with a neutron star and the subsequent process in a single simulation

Numerical simulation of a black hole-neutron star merger. The density profile is shown in blue and green, the magnetic field lines penetrating the black hole are shown in pink. Unbound matter is shown in white and its speed is indicated by green arrows. Photo credit: K. Hayashi (Kyoto University)

With supercomputer calculations, scientists from the Max Planck Institute for Gravitational Physics in Potsdam and from Japan show a consistent picture for the first time: They modeled the complete process of the collision of a black hole with a neutron star. In their studies, they calculated the process from the last orbits through the merger to the post-merger phase, in which they calculate that high-energy gamma-ray bursts can occur. The results of their studies have now been published in the journal Physical Check D.

Almost seven years have passed since the first detection of gravitational waves. On September 14, 2015, the LIGO detectors in the United States recorded the signal of two merging black holes from deep space. Since then, a total of 90 signals have been observed: from binary star systems consisting of two black holes or neutron stars, but also from mixed binary stars. If at least one neutron star is involved in the merger, there is a chance 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 (GW170817) discovered on August 17, 2017, about 70 telescopes on Earth and in space observed the electromagnetic signals. No electromagnetic counterparts to the gravitational waves have been detected in the two mergers of neutron stars with black holes observed so far (GW200105 and GW200115). But when more such events are measured with the increasingly sensitive detectors, the researchers expect electromagnetic observations here too. During and after the merger, matter is ejected from the system and electromagnetic radiation is generated. This probably also produces short gamma-ray bursts, as observed by space telescopes.

For their study, the scientists chose two different model systems, consisting of a rotating black hole and a neutron star. The masses of the black hole have been fixed at 5.4 and 8.1 solar masses, respectively, and the mass of the neutron star has been fixed at 1.35 solar masses. These parameters were chosen in such a way that a disruption of the neutron star by tidal forces was to be expected.

“We get insights into a process that lasts one to two seconds – that sounds short, but in fact a lot happens in this time: from the last orbits to the disturbance of the neutron star by tidal forces to the ejection of matter to the formation of an accretion disk the resulting black hole and the further ejection of matter in a jet,” says Masaru Shibata, Director of the Department of Computational Relativistic Astrophysics at the Max Planck Institute for Gravitational Physics in Potsdam. “This high-energy jet is probably also a reason for short gamma-ray bursts, whose origin is still a mystery. The simulation results also suggest that the ejected matter should synthesize heavy elements such as gold and platinum.”






Numerical simulation of a black hole-neutron star merger: The left side of the simulation shows the density profile (blue and green contours) with the magnetic field lines (pink curves) penetrating the black hole, unbound matter (white color) and their velocity (green arrows). The right side shows the magnetic field strength (magenta) and magnetic field lines (light blue curves). Photo credit: Max Planck Society

What happens during and after the merger?

The simulations show that the neutron star is being torn apart by tidal forces during the merger process. About 80% of the neutron star’s 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’s matter forms a one-armed spiral structure. Some of the matter in the spiral arm 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 of electromagnetic radiation that could eventually produce a brief gamma-ray burst.

Second-long simulations

The institute’s “Sakura” cluster computer took about 2 months to solve the Einstein equations for the process, which takes about two seconds. “Such general relativistic simulations are very time-consuming. For this reason, research groups worldwide have so far only concentrated on short simulations,” explains Dr. Kenta Kiuchi, group leader in Shibata’s department who developed the code. “In contrast, an end-to-end simulation, as we have now carried it out for the first time, provides a consistent picture of the entire process given binary initial conditions that are defined at the beginning.”

In addition, the researchers can only study the formation mechanism of short gamma-ray bursts, which typically last one to two seconds, with such long simulations.

Shibata and his department’s scientists are already working on similar but more complex numerical simulations to consistently model the collision of two neutron stars and the post-merger phase.


Black holes and neutron stars merge invisibly to form dense star clusters


More information:
Kota Hayashi et al, General-relativistic magnetohydrodynamic neutrino-radiation simulation of second-long black hole-neutron star mergers, Physical Check D (2022). DOI: 10.1103/PhysRevD.106.023008

Provided by the Max Planck Society

Quote: Modeling the Black Hole-Neutron Star Merger and Subsequent Process in a Single Simulation (2022, July 14), retrieved July 15, 2022 from https://phys.org/news/2022-07-merger-black -hole-neutronstar.html

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