Origin of supermassive black hole flares identified

Black holes are not always in the dark. Astronomers have spotted intense light shows shining just outside the event horizons of supermassive black holes, including the one at our galaxy’s core. However, scientists could not identify the cause of these eruptions beyond the alleged involvement of magnetic fields.

Using computer simulations of unparalleled power and resolution, physicists claim to have solved the mystery: the energy released near a black hole’s event horizon as magnetic field lines reconnect rashes, report the researchers in Letters from the Astrophysical Journal.

The new simulations show that interactions between the magnetic field and material falling into the mouth of the black hole cause the field to compress, flatten, rupture and reconnect. This process ultimately uses magnetic energy to shoot hot plasma particles at near-lightspeed into the black hole or out into space. These particles can then directly emit part of their kinetic energy in the form of photons and give nearby photons an energy boost. These energetic photons make up the mysterious black hole flares.

In this model, the disc of previously falling material is ejected during eruptions, clearing the area around the event horizon. This arrangement could provide astronomers with an unhindered view of the generally obscure processes occurring just outside the event horizon.

“The fundamental process of reconnecting magnetic field lines near the event horizon can harness magnetic energy from the black hole’s magnetosphere to fuel fast, bright flares,” says the study’s co-lead author. , Bart Ripperda, joint postdoctoral researcher at the Flatiron Institute’s Center for Computational Astrophysics (CCA) in New York and Princeton University. “That’s really where we connect plasma physics to astrophysics.”

Ripperda co-authored the new study with CCA research associate Alexander Philippov, Harvard University scientists Matthew Liska and Koushik Chatterjee, University of Amsterdam scientists Gibwa Musoke and Sera Markoff, the ‘Northwestern University Alexander Chekhovskoy and University College London scientist Ziri Younsi.

A black hole, true to its name, emits no light. Thus, flares must originate from outside the black hole’s event horizon – the boundary where the black hole’s gravitational pull becomes so strong that not even light can escape. Orbiting and falling matter surrounds black holes in the form of an accretion disk, like the one around the giant black hole found in the galaxy M87. This material cascades toward the event horizon near the black hole’s equator. At the north and south poles of some of these black holes, jets of particles shoot out into space at near the speed of light.

Identifying where flares form in a black hole’s anatomy is incredibly difficult due to the physics involved. Black holes bend time and space and are surrounded by powerful magnetic fields, radiation fields and turbulent plasma – matter so hot that electrons detach themselves from their atoms. Even with the aid of powerful computers, previous efforts could only simulate black hole systems at resolutions too low to see the mechanism that powers flares.

Ripperda and his colleagues have gone to great lengths to increase the level of detail in their simulations. They used compute 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 Popeye supercomputer at the Flatiron Institute located at the University of California. in San Diego. In total, the project required millions of hours of calculation. The result of all that computing muscle was by far the highest resolution simulation of a black hole’s environment ever, at more than 1,000 times the resolution of previous efforts.

The increased resolution gave researchers an unprecedented picture of the mechanisms leading to a black hole flare. The process centers on the black hole’s magnetic field, which has magnetic field lines that shoot out from the black hole’s event horizon, forming the jet and connecting to the accretion disk. Previous simulations have revealed that matter flowing into the black hole’s equator pulls the magnetic field lines toward the event horizon. Dragged field lines begin to pile up near the event horizon, eventually repelling and blocking incoming material.

With its exceptional resolution, the new simulation captured for the first time how the magnetic field at the boundary between flowing matter and black hole jets intensifies, compressing and flattening equatorial field lines. These field lines are now in alternating paths pointing towards the black hole or away from it. When two lines pointing in opposite directions meet, they can break, reconnect, and become tangled. Between the connection points, a pocket forms in the magnetic field. These pockets are filled with hot plasma which either falls into the black hole or is accelerated through space at enormous speeds, thanks to the energy drawn from the magnetic field in the jets.

“Without the high resolution of our simulations, you wouldn’t be able to capture subdynamics and substructures,” says Ripperda. “In low-resolution models, reconnection does not occur, so there is no mechanism that could accelerate the particles.”

The plasma particles in the catapulted material immediately emit some energy in the form of photons. Plasma particles can dive deeper into the energy range needed to give nearby photons an energy boost. These photons, whether passing or the photons initially created by the launched plasma, constitute the most energetic eruptions. The material itself ends up in a hot blob orbiting near the black hole. One such blob has been spotted near the Milky Way’s supermassive black hole. “The magnetic reconnection powering such a hotspot is a smoking gun to explain this observation,” says Ripperda.

The researchers also observed that after the black hole flares up for a while, the energy of the magnetic field decreases and the system resets. Then, over time, the process begins again. This cyclical mechanism explains why black holes emit flares on fixed schedules ranging from daily (for the supermassive black hole in our Milky Way) to every few years (for M87 and other black holes).

Ripperda believes 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 occurring and whether it alters images of a black hole’s shadow. “We’ll have to see,” Ripperda said. For now, he and his colleagues are working to improve their simulations with even more detail.

– This press release was originally published on the Simons Foundation website

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