By conducting detailed supercomputer simulations, the research team at the University of Helsinki explored the interactions among radiation, plasma, and magnetic fields surrounding black holes. Their findings indicate that turbulence driven by magnetic fields heats the nearby plasma, leading to the emission of X-rays.
Accretion Disks as X-ray Sources
A black hole forms when a massive star collapses into a dense mass, with gravity so intense that not even light can escape. As a result, black holes are observed indirectly, through their influence on the surrounding environment.
In many cases, black holes are part of a binary star system, where a companion star orbits the black hole. Matter from the companion star gradually spirals into the black hole, forming an accretion disk-a bright source of X-rays observable from Earth.
Since the 1970s, scientists have attempted to model X-ray emissions from these accretion flows. It was hypothesized that the X-rays were produced by interactions between the gas and magnetic fields, in a process 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," says Associate Professor Joonas Nattila, who leads the Computational Plasma Astrophysics research group at the University of Helsinki, focusing on extreme plasma modeling.
The Role of Radiation-Plasma Interaction
The simulations revealed that the turbulence near black holes is so intense that quantum effects significantly influence plasma dynamics.
In the simulated environment of electron-positron plasma and photons, X-ray radiation can generate electrons and positrons, which then annihilate each other, converting back into radiation.
Nattila described how these particles, typically antiparticles of each other, rarely coexist. However, the extreme energy conditions near black holes allow for such occurrences. Normally, radiation does not interact with plasma, but the high-energy photons around black holes make these interactions crucial.
"In everyday life, such quantum phenomena where matter suddenly appears in place of extremely bright light are, of course, not seen, but near black holes, they become crucial," Nattila says.
"It took us years to investigate and add to the simulations all quantum phenomena occurring in nature, but ultimately, it was worth it," he adds.
Insights into the Origins of Radiation
The study showed that turbulent plasma naturally produces the type of X-ray radiation observed from accretion disks. Additionally, the simulation revealed that plasma around black holes can exist in two distinct equilibrium states, depending on the external radiation field. In one state, the plasma is transparent and cold, while in the other, it is opaque and hot.
"The X-ray observations of black hole accretion disks show exactly the same kind of variation between the so-called soft and hard states," Nattila emphasized.
The study was published in the prestigious 'Nature Communications' journal. The simulation used is the first plasma physics model to incorporate all significant quantum interactions between radiation and plasma. This research is part of a project led by Nattila, funded by a euro 2.2 million Starting Grant from the European Research Council, which aims to deepen understanding of plasma and radiation interactions.
Research Report:Radiative plasma simulations of black hole accretion flow coronae in the hard and soft states
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