The process that causes black hole flares has been discovered by astrophysicists. The researchers discovered that flares are powered by energy released near a black hole’s event horizon during the reconnection of magnetic field lines, using computer models of unprecedented power and detail. The results point to interesting new prospects for studying the area immediately beyond a black hole’s event horizon, according to the researchers.
Black holes don’t necessarily exist in complete darkness. Astronomers have discovered bright light displays just beyond the event horizons of supermassive black holes, including the one at the center of our galaxy. Apart from the possible participation of magnetic fields, scientists have been unable to pinpoint the source of these flares.
Physicists claim to have answered the puzzle by using supercomputer simulations. Energy released near a black hole’s event horizon during the reconnection of magnetic field lines fuels the flares, the researchers write in The Astrophysical Journal Letters on January 14.
The new models indicate that the magnetic field compresses, flattens, breaks, then reconnects as it interacts with material plunging into the black hole’s mouth. Ultimately, magnetic energy is used to catapult hot plasma particles into the black hole or out into space at near-light speed. Those particles may then emit some of their kinetic energy as photons, providing an energy boost to adjacent photons. The enigmatic black hole flares are made out of those powerful photons.
In this scenario, flares release a disk of previously falling material, clearing the space surrounding the event horizon. This cleaning up might provide astronomers a clear picture of the normally hidden activities taking place close beyond the event horizon.
“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) and Princeton University in New York City. “This is where we’re truly bringing plasma physics and astrophysics together.”
Ripperda, 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 collaborated on the new study.
A black hole, as its name implies, does not emit any light. Flares must thus originate beyond the black hole’s event horizon, which is the point at which the gravitational pull of the black hole becomes so powerful that not even light can escape. An accretion disk surrounds black holes in the form of orbiting and infalling material, such as the one observed surrounding the M87 galaxy’s monster black hole. This material falls toward the black hole’s equator, where it collides with the event horizon. Jets of particles fly out into space at almost the speed of light from the north and south poles of some of these black holes.
Because of the physics involved, determining where flares emerge in a black hole’s anatomy is very challenging. Black holes are surrounded by enormous magnetic fields, radiation fields, and chaotic plasma — stuff so hot that electrons 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 discern the mechanism that drives the flares.
Ripperda and his colleagues went all out to make their simulations more detailed. They employed computing time from three supercomputers: the Oak Ridge National Laboratory’s Summit supercomputer, the University of Texas at Austin’s Longhorn supercomputer, and the Flatiron Institute’s Popeye supercomputer at the University of California, San Diego. The project used millions of computation hours in total. With almost 1,000 times the resolution of prior efforts, the outcome of all this computer muscle was by far the highest-resolution simulation of a black hole’s environs ever produced.
The researchers were able to get an unparalleled image of the events that lead to a black hole flare because to the higher resolution. The process revolves on the black hole’s magnetic field, which has magnetic field lines that emerge from the event horizon and link to the accretion disk, generating the jet. Material pouring into the black hole’s equator draws magnetic field lines toward the event horizon, according to previous models. Near the event horizon, the dragging field lines begin to build up, finally pushing back and obstructing the material pouring in.
The new simulation captures for the first time how the magnetic field develops at the boundary between streaming material and the black hole’s jets, compressing and flattening the equatorial field lines, thanks to its remarkable resolution. Those field lines are now in alternating lanes, heading either toward or away from the black hole. When two lines heading in different directions come together, they might tangle, break, and reattach. In the magnetic field, a pocket develops between connecting points. Those pockets are packed with heated plasma that either falls into the black hole or is blasted out into space at incredible speeds owing to the energy extracted from the jets’ magnetic fields.
“You couldn’t represent the subdynamics and substructures without our models’ great resolution,” Ripperda explains. “Reconnection does not occur in low-resolution models, thus there is no mechanism that may speed particles.”
Photons are instantaneously emitted by plasma particles in the hurled material. The plasma particles may go further lower into the energy range required to enhance surrounding photons. The most intense flares are made up of photons from passers-by or photons generated by the propelled plasma. The material eventually condenses into a heated glob around the black hole. Near the Milky Way’s supermassive black hole, such a blob has been discovered. “A smoking gun for understanding that observation is magnetic reconnection powering such a hot area,” Ripperda explains.
The magnetic field energy wanes when the black hole flares for a bit, and the system resets, according to the researchers. The process then repeats itself over time. This cyclical process explains why black holes produce flares on a regular basis, ranging from once a day (for our Milky Way’s supermassive black hole) to once every few years (for M87 and other black holes).
Ripperda believes that data from the James Webb Space Telescope, paired with those from the Event Horizon Telescope, will be able to establish if the process described in the new simulations is occurring, and whether it alters pictures of a black hole’s shadow. Ripperda adds, “We’ll have to see.” For the time being, he and his colleagues are focusing on making their simulations even more detailed.
The Flatiron Institute’s mission
The Simons Foundation’s research arm is the Flatiron Institute. The objective of the institution is to enhance scientific research by using computational approaches such as data analysis, theory, modeling, and simulation. The Center for Computational Astrophysics at the institution develops novel computational frameworks that enable scientists to evaluate large astronomical datasets and comprehend complicated, multi-scale physics in a cosmological context.
