Researchers have achieved a significant milestone that has fascinated scientists since the 1970s: providing an explanation for the X-ray radiation emanating from the vicinity of black holes. This radiation is generated by the chaotic movements of magnetic fields interacting with turbulent plasma gas.
Researchers at the University of Helsinki have achieved a significant milestone that has fascinated scientists since the 1970s: providing an explanation for the X-ray radiation emanating from the vicinity of black holes. This radiation is generated by the chaotic movements of magnetic fields interacting with turbulent plasma gas.
Through extensive supercomputer simulations, the team modeled how radiation interacts with plasma and magnetic fields surrounding black holes. They discovered that the turbulence caused by the magnetic fields effectively heats the nearby plasma, leading to its radiation.
Understanding X-ray radiation from accretion disks
A black hole forms when a massive star collapses into such a dense mass that its gravitational pull is strong enough to prevent even light from escaping. Because of this, black holes are not directly visible; instead, they can only be studied through their effects on surrounding matter.
Most known black holes are part of binary star systems, with a companion star that gradually loses material to the black hole. This infalling gas often creates a bright accretion disk around the black hole, which emits X-rays that we can observe.
Since the 1970s, scientists have sought to model the radiation produced by these accretion flows. During that time, it was already believed that X-rays were generated from the interaction between local gas and magnetic fields, similar to how solar flares heat the Sun’s surroundings.
“The flares in the accretion disks of black holes are like extreme versions of solar flares,” explains Associate Professor Joonas Nättilä, who leads the Computational Plasma Astrophysics research group at the University of Helsinki, focusing on such extreme plasma modeling.
Interactions between radiation and plasma
The simulations revealed that the turbulence around black holes is intense enough that quantum effects significantly influence plasma dynamics.
In the simulated environment of electron-positron plasma and photons, X-ray radiation can create electron-positron pairs which may later annihilate back into radiation upon interaction.
Nättilä notes that electrons (negatively charged) and positrons (positively charged) typically do not coexist, but the high-energy environment near black holes allows for such occurrences. Additionally, while photons usually do not interact with plasma, the extreme energy of photons around black holes alters this dynamic.
“In everyday scenarios, we don’t observe such quantum phenomena where matter emerges from intensively bright light, but near black holes, they are essential,” Nättilä asserts.
“It took us years to incorporate all the quantum phenomena in our simulations, but it ultimately proved worthwhile,” he adds.
A clearer understanding of radiation sources
The research showed that turbulent plasma naturally generates the X-ray radiation detected from accretion disks. For the first time, the simulations revealed that the plasma surrounding black holes can exist in two distinct equilibrium states, influenced by the external radiation field: one being transparent and cool, while the other is opaque and hot.
“The observed X-ray variations from black hole accretion disks align perfectly with these soft and hard states,” Nättilä points out.
The findings were published in the journal Nature Communications. This study represents the first plasma physics model to account for all critical quantum interactions between radiation and plasma. It is part of a project led by Nättilä that received a €2.2 million Starting Grant from the European Research Council, aimed at exploring the intricate relationships between plasma and radiation.
