Not all black holes are in pitch blackness. Astronomers have observed powerful light displays coming from supermassive black holes, including the one at the center of our galaxy, that are located close outside the event horizon. However, beyond the supposition that magnetic fields were involved, scientists were unable to pinpoint the cause of these flares.
By employing computer simulations of unparalleled power and resolution, physicists say they’ve solved the mystery: Energy released near a black hole’s event horizon during the reconnection of magnetic field lines powers the flares, the researchers report January 14 in The Astrophysical Journal Letters.
According to the new models, the magnetic field flattens, flattens, breaks, and then reconnects as a result of interactions with the matter falling into the black hole’s mouth. In the end, that process employs magnetic energy to shoot hot plasma particles into the black hole or out into space at close to light speed.
Then, those particles can directly emit some of their kinetic energy as photons, therefore energizing surrounding photons. The enigmatic black hole flares are made up of those powerful photons.
In this scenario, the event horizon clearing disk of previously infalling material is expelled during flares. Astronomers might have a clear glimpse of the typically hidden processes occurring close outside the event horizon as a result of this tidying up.
“The fundamental process of reconnecting magnetic field lines near the event horizon can tap the magnetic energy of the black hole’s magnetosphere to power rapid and bright flares,” says study co-lead author Bart Ripperda, a joint postdoctoral fellow at the Flatiron Institute’s Center for Computational Astrophysics (CCA) in New York City and Princeton University. “This is really where we’re connecting plasma physics with astrophysics.”
Ripperda co-authored the new study with CCA associate research scientist Alexander Philippov, Harvard University scientists Matthew Liska and Koushik Chatterjee, University of Amsterdam scientists Gibwa Musoke and Sera Markoff, Northwestern University scientist Alexander Tchekhovskoy and University College London scientist Ziri Younsi.
A black hole, true to its name, emits no light. Flares must therefore originate from outside the event horizon of a black hole, which is the barrier beyond which no light can escape due to the black hole’s intense gravitational pull.
Without the high resolution of our simulations, you couldn’t capture the subdynamics and the substructures. In the low-resolution models, reconnection doesn’t occur, so there’s no mechanism that could accelerate particles.
Bart Ripperda
Black holes are surrounded by material that is infalling and orbiting them in the form of an accretion disk, like the one that surrounds the enormous black hole discovered in the M87 galaxy. Near the equator of the black hole, this material cascades in the direction of the event horizon. Particle jets that leave the north and south poles of some of these black holes almost travel at the speed of light.
Because of the physics involved, it is very challenging to pinpoint where the flares originate within a black hole’s anatomy. Time and space are bent by black holes, which are also surrounded by intense magnetic and radiation fields as well as chaotic plasma matter that is so intense that electrons can separate from their atoms. Previous attempts, despite the use of sophisticated computers, could only mimic black hole systems at resolutions that were too low to see the mechanism driving the flares.
Ripperda and his colleagues went all in on boosting the level of detail in their simulations. They used computing time on three supercomputers the Summit supercomputer at Oak Ridge National Laboratory in Tennessee, the Longhorn supercomputer at the University of Texas at Austin, and the Flatiron Institute’s Popeye supercomputer located at the University of California, San Diego.
In total, the project took millions of computing hours. With almost 1,000 times the resolution of prior attempts, the end result of all this processing power was by far the highest-resolution simulation of a black hole’s environs ever created.
The improved resolution provided the researchers with a previously unattainable view of the processes leading to a black hole flare. The process is centered on the magnetic field of the black hole, which has magnetic field lines that extend from the event horizon of the black hole, create the jet, and link to the accretion disk.
Previous simulations revealed that material flowing into the black hole’s equator drags magnetic field lines toward the event horizon. The dragged field lines begin stacking up near the event horizon, eventually pushing back and blocking the material flowing in.
With its remarkable resolution, the new simulation successfully depicted for the first time how the equatorial field lines become compressed and flattened when the magnetic field at the boundary between the streaming material and the black hole jets intensifies. Those field lines are currently arranged in alternating lanes that point either away from or toward the black hole. Two lines can break, reattach, and tangle when they cross when they are pointing in different directions.
In between connection points, a pocket forms in the magnetic field. Those pockets are filled with hot plasma that either falls into the black hole or is accelerated out into space at tremendous speeds, thanks to energy taken from the magnetic field in the jets.
“Without the high resolution of our simulations, you couldn’t capture the subdynamics and the substructures,” Ripperda says. “In the low-resolution models, reconnection doesn’t occur, so there’s no mechanism that could accelerate particles.”
Plasma particles in the catapulted material immediately radiate some energy away as photons. The plasma particles can further dip into the energy range needed to give nearby photons an energy boost. Those photons, either passersby or the photons initially created by the launched plasma, make up the most energetic flares. The material itself ends up in a hot blob orbiting in the vicinity of the black hole. Such a blob has been spotted near the Milky Way’s supermassive black hole.
“Magnetic reconnection powering such a hot spot is a smoking gun for explaining that observation,” Ripperda says.
The researchers also observed that after the black hole flares for a while, the magnetic field energy wanes, and the system resets. Then, over time, the process begins anew. This cyclical mechanism explains why black holes emit flares on set schedules ranging from every day (for our Milky Way’s supermassive black hole) to every few years (for M87 and other black holes).
Ripperda thinks that observations from the recently launched James Webb Space Telescope combined with those from the Event Horizon Telescope could confirm whether the process seen in the new simulations is happening and if it changes images of a black hole’s shadow.
“We’ll have to see,” Ripperda says. For now, he and his colleagues are working to improve their simulations with even more detail.
About the Flatiron Institute
The Flatiron Institute is the research division of the Simons Foundation. The institute’s mission is to advance scientific research through computational methods, including data analysis, theory, modeling, and simulation. The institute’s Center for Computational Astrophysics creates new computational frameworks that allow scientists to analyze big astronomical datasets and to understand complex, multi-scale physics in a cosmological context.
