Tangled magnetic fields may power jets from supermassive black holes

A composite image of the active galaxy Centaurus A shows huge jets extending millions of light years into space, presumably fired out from a central supermassive black hole. Image: Optical: ESO/WFI; Submillimeter: MPIfR/ESO/APEX/A.Weiss et al.; X-ray: NASA/CXC/CfA/R.Kraft et al.

Lurking in the cores of active galaxies are supermassive black holes that can blast out enormous jets of charged particles stretching millions of light years into space. The processes that generate those jets may be responsible for cosmic rays that carry millions of times the energy produced in the most powerful particle accelerators on Earth.

Exactly what mechanism powers those galactic jets is not known. But a new computer simulation at the SLAC National Accelerator Laboratory in Menlo Park, California, indicates distorted magnetic fields, tangled like spaghetti, create powerful electric fields in the direction of the jets, setting up dense currents of high-energy particles streaming away into space.

Using the Mira supercomputer at the Argonne National Laboratory, the researchers simulated the motions of up to 550 billion particles, mimicking a cosmic jet, and then scaled up the results to compare with actual observations. The simulations showed that when a helical magnetic field, created by a spinning supermassive black hole, is distorted it becomes highly tangled, producing a powerful electric field inside the jet.

Computer simulations of cosmic jets indicate when the jet’s helical magnetic field is distorted, the magnetic field lines get tangled, generating an electric field inside the jet that can accelerate charged particles to the energies observed in actual jets from active galaxies. Image: Greg Stewart/SLAC National Accelerator Laboratory

The electric and magnetic fields can accelerate protons and electrons to enormous velocities. High-energy electrons radiate their energy away in X-rays and gamma rays, but protons can escape into space and eventually reach Earth as cosmic rays.

“We see that a large portion of the magnetic energy released in the process goes into high-energy particles, and the acceleration mechanism can explain both the high-energy radiation coming from active galaxies and the highest cosmic-ray energies observed,” said Paulo Alves, lead author of a paper in Physical Review Letters.

Roger Blandford, former director of the SLAC/Stanford University Kavli Institute for Particle Astrophysics and Cosmology, said the simulations identify “many surprising details of what happens under conditions thought to be present in distant jets, and may help explain some remarkable astrophysical observations.”