From cosmic merger to gamma-ray burst in a single model

July 15 () –

Using supercomputer calculations, scientists at the Max Planck Institute for Gravitational Physics have modeled the entire process of the collision of a black hole with a neutron star.

In their studies, they calculated the process from the final orbits through the merger to the post-merger phase in which, according to their calculations, bursts of high-energy gamma rays can occur. The results of their studies have now been published in the journal Physical Review D.

Almost seven years have passed since the first detection of gravitational waves. On September 14, 2015, LIGO detectors in the US recorded the signal from two merging black holes from deep in 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, 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 detected on August 17, 2017 (GW170817), some 70 telescopes on Earth and in space observed the electromagnetic signals.

In the two neutron star-black hole mergers observed so far (GW200105 and GW200115), no electromagnetic equivalents of gravitational waves were detected. But when more such events are measured with increasingly sensitive detectors, the researchers expect electromagnetic observations here as well. During and after the merger, matter is expelled from the system and electromagnetic radiation is generated. This probably also produces brief bursts of gamma rays, 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 were set at 5.4 and 8.1 solar masses, respectively, and the mass of the neutron star was set at 1.35 solar masses. These parameters were chosen so that the neutron star could be expected to be ripped apart by tidal forces.

“We get information about a process that lasts a second or two; it seems short, but in fact a lot happens during that time: from the final orbits and the disruption of the neutron star by tidal forces, the ejection of matter, until the formation of an accretion disk around the nascent black hole and further ejection of matter in a jetsays Masaru Shibata, head of the Department of Computational Relativistic Astrophysics at the Max Planck Institute for Gravitational Physics in Potsdam.

“This high-energy jet is probably also the reason for the brief gamma-ray bursts, the origin of which is still mysterious. The simulation results also indicate that the ejected matter should synthesize heavy elements like gold and platinum.”

The simulations show that during the merger process, the neutron star is torn apart by tidal forces. About 80% of the neutron star’s matter falls into the black hole in a few milliseconds, increasing its mass by about one solar mass. In the next 10 milliseconds, the neutron star’s matter forms a single-armed spiral structure. Some of the matter in the spiral arm is expelled 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 the merger, this causes a stream of focused electromagnetic radiation in the form of a jet, which could ultimately produce a brief burst of gamma rays.

It took about 2 months for the “Sakura” department’s cluster computer to solve Einstein’s equations for the process that takes about two seconds. “These general relativistic simulations are very time consuming. That is why research groups around the world have so far focused only on short simulations,” he explains. it’s a statement Dr. Kenta Kiuchi, group leader in Shibata’s department, who developed the code. “By contrast, an end-to-end simulation, like the one we’ve done now for the first time, provides a self-consistent picture of the entire process for given binary initial conditions that are defined once at the beginning”.

Furthermore, it is only with such long simulations that researchers can explore the mechanism behind the generation of short gamma-ray bursts, which typically last one or 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 post-merger phase.