How black holes produce powerful relativistic jets

A hundred years before the Event Horizon Telescope Collaboration released the first image of a black hole in 2019—located at the heart of the galaxy M87—astronomer Heber Curtis had already discovered a strange jet protruding from the galaxy’s center. Today, we know this to be the jet of the black hole M87*. Such jets are also emitted by other black holes. Theoretical astrophysicists at Goethe University have now developed a numerical code to describe with high mathematical precision how black holes transform their rotational energy into such ultra-fast jets.

The findings are published in The Astrophysical Journal Letters.

For nearly two centuries, it was unclear that the bright spot in the constellation Virgo, which Charles Messier had described in 1781 as “87: Nebula without stars,” was in fact a very large galaxy. As a result, there was initially no explanation for the strange jet discovered in 1918 emerging from the center of this “nebula.”

At the heart of the giant galaxy M87 lies the black hole M87*, which contains a staggering six and a half billion solar masses and spins rapidly on its axis. Using the energy from this rotation, M87* powers a particle jet expelled at nearly the speed of light, stretching across an immense 5,000 light-years. Such jets are also generated by other rotating black holes. They contribute to disperse energy and matter throughout the universe and can influence the evolution of entire galaxies.

A team of astrophysicists at Goethe University Frankfurt, led by Prof. Luciano Rezzolla, has developed a numerical code, named the Frankfurt particle-in-cell code for black hole spacetimes (FPIC), which describes with high precision the processes that convert rotational energy into a particle jet.

The result: In addition to the Blandford–Znajek mechanism—which has so far been considered responsible for the extraction of rotational energy from the black hole via strong magnetic fields—the scientists have revealed that another process is involved in the energy extraction, namely, magnetic reconnection. In this process, magnetic field lines break and reassemble, leading to magnetic energy being converted into heat, radiation, and eruptions of plasma.

The FPIC code simulated the evolution of a vast number of charged particles and extreme electromagnetic fields under the influence of the black hole’s strong gravity. Dr. Claudio Meringolo, the main developer of the code, explains: “Simulating such processes is crucial for understanding the complex dynamics of relativistic plasmas in curved spacetimes near compact objects, which are governed by the interplay of extreme gravitational and magnetic fields.”

The investigations required highly demanding supercomputer simulations that consumed millions of CPU hours on Frankfurt’s “Goethe” supercomputer and Stuttgart’s “Hawk.” This large computing power was essential to solve Maxwell’s equations and the equations of motion for electrons and positrons according to Albert Einstein’s theory of general relativity.

In the equatorial plane of the black hole, the researchers’ calculations revealed intense reconnection activity, leading to the formation of a chain of plasmoids—a condensation of plasma in energetic “bubbles”—moving at nearly the speed of light. According to the scientists, this process is accompanied by the generation of particles with negative energy that is used to power extreme astrophysical phenomena like jets and plasma eruptions.

“Our results open up the fascinating possibility that the Blandford–Znajek mechanism is not the only astrophysical process capable of extracting rotational energy from a black hole,” says Dr. Filippo Camilloni, who also worked on the FPIC project, “but that magnetic reconnection also contributes.”

“With our work, we can demonstrate how energy is efficiently extracted from rotating black holes and channeled into jets,” says Rezzolla. “This allows us to help explain the extreme luminosities of active galactic nuclei as well as the acceleration of particles to nearly the speed of light.”

He adds that it is incredibly exciting and fascinating to better understand what happens near a black hole using sophisticated numerical codes. “At the same time, it is even more rewarding to be able to explain the results of these complex simulations with a rigorous mathematical treatment—as we have done in our work.”