Black holes may leak more energy to their surroundings than previously suspected — and the faster these voids spin, the more efficient this energy extraction seems to be.
With this in mind, a team of scientists has discovered how the disks of gas and dust that swirl around black holes can become the powerful engines of galactic power plants. Here’s what that means.
Since 1977, researchers have theorized that energy is primarily drawn from the spins of black holes due to the objects’ magnetic fields, and funneled into powerful high-energy particle jets that erupt from the objects’ poles by a process called the “Blandford-Znajek (BZ) effect.” However, scientists haven’t been sure about many things surrounding this process, such as what determines the amount of energy that gets converted.
In an attempt to tackle these questions, a research team simulated the action of a so-called accretion disk around a supermassive black hole; not only could this simulation grant crucial insights into the complex physics around black holes, but it has the power to potentially redefine our understanding of the role supermassive black holes play in shaping entire galaxies.
“It’s long been known that infalling gas can extract spin energy from a black hole,” Jason Dexter, team member and a researcher at the University of Colorado, Boulder said in a statement. “Usually, we assume this is important for powering jets.”
By making more precise measurements, Dexter said, his team’s new research suggests a lot more energy gets extracted from black holes than was previously known.
“This energy could be radiated away as light, or it could cause gas to flow outwards,” Dexter continued. “Either way, extracted spin energy could be an important energy source for lighting up the regions near the black hole event horizon.”
Scientists have been studying black holes and their interactions with surrounding galaxies for decades. The aim has been to discover how the supermassive black holes at the hearts of galaxies, which have masses millions or billions of times that of the sun, power active galactic nuclei (AGNs) and near light-speed jets. AGNs are often so bright they outshine the combined light of every star in the galaxies around them, and that requires a lot of energy — energy that has to come from somewhere.
Many of these prior studies have focused on low-luminosity sources with sphere-like “accretion flows” feeding the black holes. That is because it has been challenging to model the theoretically unstable and thin, yet very dense and highly magnetized disks in higher-luminosity AGNs.
What research has been carried out on these systems has suggested that strong magnetic fields may help to stabilize these disks, but if that is the case, it’s unclear what role those fields then play in energy extraction and jet creation.
“We wanted to understand how energy extraction works in these highly magnetized environments,” Prasun Dhang, team member and a postdoctoral researcher at the University of Colorado, Boulder, said in the statement.
The advanced computer model employed by the team, named the 3D general relativistic magnetohydrodynamic (GRMHD), simulates the physics of superheated gas, or “plasma,” in the curved fabric of spacetime and high-gravity region around black holes.
This allowed the researchers to observe how magnetic fields interacted with black holes spinning at different speeds, specifically looking at the efficiency of energy extraction.
“The goal was to see how magnetic flux threading the black hole impacts energy extraction and whether it leads to the formation of jets,” Dhang said.
The simulations revealed that between 10% and 70% of the energy extracted from the spin of black holes was channeled to its jets via the BZ process.
“The higher [faster] the spin, the more energy the black hole can release,” Dhang continued.
The rest of the energy extracted from the spin of the black hole (but not channeled to jets) was either absorbed by the accretion disk or was dissipated as heat.
The team also found that the strength of the magnetic field increased the brightness of the black hole’s accretion disk. That could explain why some AGNs are far brighter than predicted by theoretical models.
“The unused energy close to the black hole could heat the disk and contribute to a corona,” Dhang said.
The team now intends to conduct further simulations and better understand how coronas could form around black holes.
The team’s research was published on Feb. 14 in The Astrophysical Journal.
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