The Fast and the Furious

The Crab Nebula (shown here) was created by a supernova and its spinning-neutron star remnant known as a pulsar. Pulsar wind nebulae have a high content of charged particles traveling at near the speed of light. Acceleration of these particles is likely caused by turbulence in the plasmas. Hubble Space Telescope mosaic image assembled from 24 individual Wide Field and Planetary Camera 2 exposures taken in October 1999, January 2000, and December 2000 (NASA).

The lovely Crab Nebula was created by a supernova and its spinning-neutron-star remnant known as a pulsar. Pulsar wind nebulae, such as the Crab, shine because they contain plasmas of charged particles, such as electrons and positrons, traveling at near the speed of light. A key question in astrophysics has long been: What process accelerates some of the charged particles in plasmas to energies much higher than the average particle energy, giving them near light speeds? It can’t be collisions because the particles are simply too far apart to run into each other. It can’t just be high temperature, because cold plasma can also contain a small population of particles that move at a significant fraction of the speed of light. But, it must be something fairly ordinary, because relativistic plasmas are not uncommon.

Research associate Vladimir Zhdankin, Fellow Mitch Begelman and their University of Colorado Boulder colleagues believe they have found the answer.

“The physical mechanism responsible for accelerating these particles may have been hiding in plain sight,” Zhdankin explained. “The turbulence that inevitably forms in astrophysical plasmas can be an efficient particle accelerator.”

To test this idea, the researchers performed a massive supercomputer-based simulation of the effects of turbulence on relativistic plasmas in which the particles do not collide, but rather interact through their mutual electric and magnetic fields. The simulation showed how the energy of large-scale motions and magnetic fields starts a process that leads to a fraction of the particles gaining a massive amount of energy. As the turbulent motions approach relativistic speeds, particle acceleration becomes more efficient––sufficiently efficient to explain the acceleration of the charged particles in the plasma to near light speeds.

Electric current density in a simulation of turbulence in relativistic plasmas.

Image Credit
Vladimir Zhdankin / JILA

The researchers also found evidence that particle acceleration may also be the result of the random scattering of particles between turbulent magnetic structures, a mechanism originally proposed by Enrico Fermi in 1949. This mechanism suggests that turbulence may produce energetic particles in a variety of high-energy astrophysical systems.

Either way, Zhdankin says that turbulence has a big advantage over other proposed mechanisms, such as shockwaves and magnetic reconnection, as the main mechanism for accelerating charged particles to relativistic speeds in plasmas. Turbulence is likely to be inevitable in astrophysical plasmas. In contrast, the alternatives mechanisms require specific large-scale configurations that may not be always present.

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