Black holes are famous for sucking in everything that crosses their event horizons, including light. So, why do astronomers see energetic radiation coming from the environment of a black hole in an X-ray binary system? It’s a good question that finally has an answer.
As a black hole and its companion star in the system orbit in a mutual gravitational dance, material from the star spirals toward the black hole. It forms an accretion disk which glows bright in X-rays. The disk is threaded through by strong magnetic fields that get twisted as the black hole and disk spins. But, where do the X-rays originate? It turns out they stream from turbulent regions in the disk. They don’t come from the black hole itself.
To understand these binary systems better, it helps to take a general look at their origins. These odd couples generally contain a regular star (usually a main-sequence one) coupled gravitationally to a neutron star or a black hole. There are several types of systems. One is the low-mass type with a star that has a lower mass than the neutron star or black hole companion. There are intermediate-mass ones, which contain an intermediate-mass star, and the high-mass x-ray binary that has a very high-mass star in the system.
Artist’s impression of an X-ray binary system. This one is called MAXI J1820+070, with a black hole (small black dot at the center of the gaseous accretion disk) and a companion star. Image produced with Binsim (credit: R. Hynes).
The black hole/neutron star components form when a supermassive companion star explodes as a supernova. After that, the donor star starts losing mass to the dead star companion. The infalling material generally creates the accretion disk where high-energy activity takes place. Generally, the action in the accretion disk generates the emissions astronomers detect in these systems. The low-mass binaries emit more X-rays as part of their radiation “budget”, while the high-mass ones emit a lot of optical light in addition to the X-rays.
For a long time, scientists tried to understand the sources of the high-energy radiation by watching as the material was swept into the accretion disks. X-rays generally occur in extremely energetic environments. So, everyone assumed that these disks had localized energetic regions. One idea was that magnetic fields and local gas clouds interacted and that generated the x-rays. The activity looks similar to heating in the Sun’s environment created by magnetic activity related to solar flares. Flares do occur in the accretion disks around black holes, and they’re much more extreme than our Sun’s outbursts.
Supercomputer simulations done at the University of Helsinki helped pinpoint the cause of the X-rays. They modeled interactions between radiation, superheated plasma, and magnetic fields in black hole accretion disks in binary pairs. The simulations showed that the turbulence around the black hole is incredibly strong. The plasma actually does produce X-rays emanating from accretion disks. Joonas Nättilä of the Computational Plasma Astrophysics group at the university led a team that investigated this kind of extreme plasma. He pointed out that to understand what’s happening we have to look at the effects of quantum electrodynamics on the system.
The team modeled a mix of electron-positron plasma and photons. Electron-positron plasma is a state where electrons and positrons interact in the confines of a strong magnetic field. In such conditions, the local X-ray radiation turns into electrons and positrons. Then, they annihilate back into radiation as they re-establish contact. Electrons and positrons are antiparticles of each other. That means they don’t usually occur in the same place. In addition, plasma and radiation don’t usually interact with each other. But, that can all change when you get into the environment around a black hole. There, electrons and positrons exist in close quarters and photons become so energetic that they become part of the activity.
“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,” Nättilä said. “It took us years to investigate and add to the simulations all quantum phenomena occurring in nature, but ultimately, it was worth it,” he added.
Explanation Found for X-ray radiation from Black Holes
Radiative Plasma Simulations of Black Hole Accretion Flow Coronae in the Hard and Soft states
