Origin of supermassive black hole flares identified: Largest-ever
simulations suggest flickering powered by magnetic 'reconnection'

Date:
February 3, 2022
Source:
Simons Foundation
Summary:
Astrophysicists have identified the mechanism that powers black hole
flares. By employing computer simulations of unparalleled power and
resolution, the researchers found that energy released near a black
hole's event horizon during the reconnection of magnetic field lines
powers the flares. The findings hint at exciting new possibilities
for observing the region just outside a black hole's event horizon.



FULL STORY ========================================================================== Black holes aren't always in the dark. Astronomers have spotted intense
light shows shining from just outside the event horizon of supermassive
black holes, including the one at our galaxy's core. However, scientists couldn't identify the cause of these flares beyond the suspected
involvement of magnetic fields.


==========================================================================
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.

The new simulations show that interactions between the magnetic field and material falling into the black hole's maw cause the field to compress, flatten, break and reconnect. That process ultimately uses magnetic energy
to slingshot hot plasma particles at near light speed into the black
hole or out into space. Those particles can then directly radiate away
some of their kinetic energy as photons and give nearby photons an energy boost. Those energetic photons make up the mysterious black hole flares.

In this model, the disk of previously infalling material is ejected
during flares, clearing the area around the event horizon. This tidying
up could provide astronomers an unhindered view of the usually obscured processes happening just outside 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) 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. So 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 infalling material surrounds black holes in
the form of an accretion disk, like the one around the behemoth black
hole found in the M87 galaxy. 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
nearly the speed of light.

Identifying where the flares form in a black hole's anatomy is incredibly difficult because of 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 from their
atoms. Even with the help of powerful computers, previous efforts could
only simulate black hole systems at resolutions too low to see the
mechanism that powers 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. The result of all this computational muscle was by far the highest-resolution simulation of a black hole's surroundings ever made,
with over 1,000 times the resolution of previous efforts.

The increased resolution gave the 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 spring
out from the black hole's event horizon, forming the jet and connecting
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 exceptional resolution, the new simulation for the first
time captured how the magnetic field at the border between the flowing
material and the black hole's jets intensifies, squeezing and flattening
the equatorial field lines.

Those field lines are now in alternating lanes pointing toward the black
hole or away from it. When two lines pointing in opposite directions
meet, they can break, reconnect and tangle. 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.

========================================================================== Story Source: Materials provided by Simons_Foundation. Original written
by Thomas Sumner.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. B. Ripperda, M. Liska, K. Chatterjee, G. Musoke, A. A. Philippov,
S. B.

Markoff, A. Tchekhovskoy, Z. Younsi. Black Hole Flares:
Ejection of Accreted Magnetic Flux through 3D Plasmoid-mediated
Reconnection. The Astrophysical Journal Letters, 2022; 924 (2):
L32 DOI: 10.3847/2041-8213/ ac46a1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220203161225.htm

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