Here’s What Drives Radiation Around Black Holes

James Maynard
Nov 27, 2019 · 4 min read

The densest objects in the Universe — black holes and neutron stars — are surrounded by vast regions of radiation. Now, a new study may have uncovered the origin of this enigmatic energy.

Black holes and neutron stars are the densest objects known to astrophysicists, and these objects also have other characteristics in common, including the fact that each of these types of bodies are surrounded by vast regions of powerful radiation. Astrophysicists have long believed that this radiation is the result of electrons moving in curved lines around these bodies, but the mechanism which powers this behavior remained mysterious.

A new study from Columbia University has now revealed that this radiation may be driven by powerful magnetic fields which snap and rearrange themselves in the chaotic environment of gas and dust surrounding these objects.

Black holes come in many sizes, and are surrounded by powerful fields of radiation. Image credit: NASA/Sofia/Lynette Cook

A team led by astrophysicists Luca Comisso and Lorenzo Sironi utilized super-computers to develop simulations to model this phenomenon. They found that a combination of chaotic motion and reconnections of powerful magnetic fields could result in the massive radiation seen from these super-dense objects.

“Turbulence and magnetic reconnection — a process in which magnetic field lines tear and rapidly reconnect — conspire together to accelerate particles, boosting them to velocities that approach the speed of light,” Comisso stated.

Flying Through the Clouds

The regions around black holes and neutron stars are filled with hot gases made up from charged particles, which can affect — and be affected by- magnetic fields. This chaotic behavior of the gas grabs magnetic field lines, driving magnetic reconnection, resulting in the powerful fields of radiation seen around these objects.

“We used the most precise technique — the particle-in-cell method — for calculating the trajectories of hundreds of billions of charged particles that self-consistently dictate the electromagnetic fields. And it is this electromagnetic field that tells them how to move,” said Sironi, assistant professor of astronomy at Columbia.

The simulations showed the particles gained most of their energy as they bounced around the magnetic fields at high velocities. These powerful magnetic fields, in turn, bending of the path of the particles, releasing radiation.

While low-energy travel parallel to the magnetic field lines, high-energy particles travel at right angles to these lines of force. The acceleration of the particles is dependent on the strength of the magnetic field surrounding the black hole or neutron star.

“This is indeed the radiation emitted around black holes and neutron stars that make them shine, a phenomenon we can observe on Earth,” Sironi said.

A colorful cloud of gas and dust in space.
A colorful cloud of gas and dust in space.
The explosion of the Crab Nebula (see on Earth almost 1,000 years ago) left behind a neutron star near its center. Image credit: NASA, ESA, J. Hester (Arizona State University)

Nearly all galaxies are centered around supermassive black holes, which can contain millions or billions of times as much mass as our Sun. However, even these behemoth objects are affected by their magnetic fields. In the galaxy Cygnus A, magnetic field lines may be holding clouds of gas and dust near the black hole, feeding it. At the center of our own galaxy, magnetism may have the opposite effect, keeping the supermassive black hole at the center of our galaxy fairly quiet.

Physicists Should Get a Charge Out of This

By better understanding how magnetic field lines interact with charged particles, the researchers hope to better understand how black holes and neutron stars behave with the medium surrounding them.

Stars spend most of their lives balanced between pressure from nuclear reactions at their cores trying to push the star apart, and gravity pulling the mass of the star inward towards its center. When the star runs out of available fuels, the star contracts.

A majority of black holes, as well as neutron stars, form from the collapse of massive stars. Atoms in stars (and everywhere) are composed of a nucleus of positively-charged protons and neutrons (which do not possess a charge), surrounded by one or more clouds of negatively-charged electrons.

Neutron stars are formed when a massive star collapses with a gravitational force great enough to shatter the electron clouds in the atoms, merging the negatively-charge particles with protons, forming neutrons.

A look at how neutron stars form and behave, from directors of the Chandra X-Ray Telescope.

“If the core of the collapsing star is between about 1 and 3 solar masses, these newly-created neutrons can stop the collapse, leaving behind a neutron star. (Stars with higher masses will continue to collapse into stellar-mass black holes.)” NASA explains.

By the time a neutron star has finished collapsing, the object can hold more mass than is found in the Sun, pressed into an area the size of the island of Manhattan. The material which makes up neutron stars is so dense, a thimbleful of one of these stellar corpses would weigh more than Mount Everest.

Gravity from the most massive stars is too great for even neutron stars to remain stable, and the object continues its inevitable collapse into a black hole. When this happens, not even light is able to escape from its surface.

Both types of objects, however, develop highly-energetic magnetic fields which have effects on the material surrounding them.

As a follow-up to this study, the team intends to investigate the Crab Nebula, the remnants of a star which exploded in a supernova, seen on Earth in the year 1054.

Analysis of the study was published in The Astrophysical Journal.

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James Maynard

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Writing about space since I was 10, still not Carl Sagan. Mailing List/Podcast:

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