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Max Planck Institute for Gravitational Physics (Albert Einstein Institute)
 1. Home
 2. News
 3. From “i” for “inspiral” to “g” for “gamma-ray burst”


FROM “I” FOR “INSPIRAL” TO “G” FOR “GAMMA-RAY BURST”

For the first time, scientists model both the merger of a black hole with a
neutron star and the subsequent process in one single simulation.


July 12, 2022

Using supercomputer calculations, scientists at 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
final orbits through the merger to the post-merger phase, in which according to
their calculations high-energy gamma-ray bursts may 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, the LIGO detectors in the USA recorded the signal of two
merging black holes 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, 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), about
70 telescopes on Earth and in space observed the electromagnetic signals. In the
two mergers of neutron stars with black holes observed so far (GW200105 and
GW200115), no electromagnetic counterparts to the gravitational waves were
detected. But when more such events are measured with the increasingly sensitive
detectors, the researchers expect electromagnetic observations here as well.
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 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 torn apart by tidal forces.

"We get insights into a process that lasts one to two seconds – that sounds
short, but in fact a lot happens during that time: from the final orbits and the
disruption of the neutron star by the tidal forces, the ejection of matter, to
the formation of an accretion disk around the nascent black hole, and 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 mysterious. The
simulation results also indicate that the ejected matter should synthesize heavy
elements such as gold and platinum."


WHAT HAPPENS DURING AND AFTER THE MERGER?

The simulations show that during the merger 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 subsequent about 10 milliseconds, the neutron star matter forms a one-armed
spiral structure. Part 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 the
merger, this causes a focused jet-like stream of electromegnetic radiation,
which could ultimately produce a short gamma-ray burst.



Numerical simulation of a black hole-neutron star merger. The density profile is
shown in blue and green, the magnetic-… [more]

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 its velocity by green arrows.

[less]

© K. Hayashi (Kyoto University)

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 its velocity by green arrows.
© K. Hayashi (Kyoto University)


SECONDS-LONG SIMULATIONS

It took the department's cluster computer "Sakura" about 2 months to solve
Einstein's equations for the process which takes about two seconds. "Such
general relativistic simulations are very time-consuming. That's why research
groups around the world have so far focused only on short simulations," explains
Dr. Kenta Kiuchi, group leader in Shibata’s department, who developed the code.
"In contrast, an end-to-end simulation, such as the one we have now performed
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."

Moreover, only with such long simulations the researchers can explore the
generation mechanism of short gamma-ray bursts, which typically last one to two
seconds.

Shibata and the 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 the merger.

 

© K. Hayashi (Kyoto University)

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 its velocity (green arrows). The right
side displays the magnetic-field strength (magenta) and magnetic-field lines
(light-blue curves).
Media contact

DR. ELKE MÜLLER

Press Officer AEI Potsdam, Scientific Coordinator +49 331 567-7303 +49 331
567-7298 elke.mueller@...

Science contacts

PROF. MASARU SHIBATA

Director +49 331 567-7222 +49 331 567-7298 masaru.shibata@...


DR. KENTA KIUCHI

Group Leader +49 331 567-7323 kenta.kiuchi@...

Publication
1.
Hayashi, K.; Fujibayashi, S.; Kiuchi, K.; Kyutoku, K.; Sekiguchi, Y; Shibata, M.
General-relativistic neutrino-radiation magnetohydrodynamic simulation of
seconds-long black hole-neutron star mergers
Physical Review D 106, 023008 (2022)
Source
DOI

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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 its velocity by green arrows.
© K. Hayashi (Kyoto University)
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