The behavior of black hole jets on Earth

Dive into the heart of an active galaxy and you’ll find a supermassive black hole gobbling up material from its surroundings. In about one in ten such galaxies, the black hole will also emit streams of matter at speeds close to the speed of light. Such relativistic black holes are thought to contain, among other ingredients, a plasma of electron pairs and their antimatter equivalents, positrons.

This relativistic electron-positron plasma is believed to shape the dynamics and energy budget of the black hole and its environment. But how exactly this happens remains poorly understood, because it is difficult to measure the plasma with astronomical observations and to simulate it with computer programs.

In a paper recently published in Nature CommunicationsCharles Arrowsmith and colleagues from the Fireball collaboration report how they have used the HiRadMat facility at CERN to produce a relativistic electron-positron plasma beam that allows this medium to be studied in detail in laboratory experiments.

Relativistic beams of electron-positron pairs can be created in several ways in different types of laboratories, including high-power laser equipment. However, none of the existing methods can produce the number of electron-positron pairs required to maintain a plasma – a state of matter in which the constituent particles are very loosely bound. Without supporting plasma, researchers cannot investigate how these analogs of black hole jets change as they move through a laboratory equivalent of the interstellar medium. This investigation is key to explaining observations from ground-based and space-based telescopes.

Arrowsmith and colleagues found a way to meet these requirements at CERN’s HiRadMat facility. Their approach involved extracting three hundred billion protons from the Laboratory’s Super Proton Synchrotron within a nanosecond and throwing them at a graphite and tantalum target, in which a cascade of particle interactions generates a large number of electron-positron pairs.

By measuring the resulting relativistic electron-positron beam with an array of instruments and comparing the result with sophisticated computer simulations, Arrowsmith and colleagues showed that the number of electron-positron pairs in the beam—more than ten trillion—is ten to hundreds of times greater. larger than previously achieved, exceeding for the first time the number needed to maintain the plasma state.

“Electron-positron plasmas are thought to play a fundamental role in astrophysical jets, but computer simulations of these plasmas and jets have never been tested in the laboratory,” says Arrowsmith. “Laboratory experiments are necessary to validate the simulations, because what appear to be reasonable simplifications of the calculations involved in the simulations can sometimes lead to drastically different conclusions.”

The result is the first of a series of experiments the Fireball collaboration is conducting at HiRadMat.

“The basic idea of ​​these experiments is to reproduce in the laboratory the microphysics of astrophysical phenomena such as jets from black holes and neutron stars,” says co-author of the paper and lead researcher Gianluca Gregori. “What we know about these phenomena comes almost exclusively from astronomical observations and computer simulations, but telescopes can’t really probe the microphysics, and the simulations involve approximations. Laboratory experiments like these are a bridge between these two approaches.”

Next in plasma research by Arrowsmith and colleagues at HiRadMat is to spread these powerful jets through a meter-long plasma and observe how the interaction between them generates magnetic fields that accelerate particles in the jets—one of the biggest puzzles in energy. up. astrophysics.

“The Fireball experiments are one of the latest additions to the HiRadMat portfolio,” says facility operations manager Alice Goillot. “We look forward to continuing to reproduce these rare phenomena using the unique properties of the CERN accelerator complex.”

This project has received funding from the European Union’s Horizon Europe Research and Innovation program under Grant Agreement No. 101057511 (EURO-LABS).