& bullet; Physics 11, 130
Large-scale simulations suggest a mechanism by which supermassive black holes can accelerate particles to ultrahigh energies.
For decades, astrophysicists have hypothesized that the mysterious, ultra-high-energy cosmic rays (UHECRs) found on Earth could come from active galactic nuclei (AGNs), the supermassive black holes in the centers of galaxies. However, the particle acceleration mechanism was unclear. Now a research team has proposed a mechanism with simulations of unprecedented size. The results reinforce the idea of AGNs as UHECR sources, although some experts warn that the simulations have limitations when used to predict protons of such high energies.
Most cosmic rays are protons or atomic nuclei with energies around
. UHECRs have energies above
and have been sporadically discovered on Earth since the 1960s, although their sources are uncertain. A recent coincident detection of a high-energy neutrino and a gamma-ray from an AGN was considered strong evidence for AGNs as UHECR sources, as both events can be explained by the acceleration of protons to ultrahigh energies . Other candidates for UHECR sources, such as relativistic supernovae and gamma-ray bursts, are still feasible.
Astrophysical rays are the rays of ionized matter emitted in both directions along the axis of rotation of a massive astronomical object, such as AGN. Relativistic AGN jets, with some particles moving near the speed of light, have been thoroughly studied both observationally and theoretically. Most of their energy is stored in strong magnetic fields that assume a helical structure. Earlier simulations have shown that the jet plasma is not stable. Interstellar medium interactions cause distortions of the jet called kink instabilities, with the columnar shape of the jet bending and twisting.
The researchers hypothesized that these kink instabilities can generate electric fields that are strong enough to accelerate particles to ultra-high energies and even have gamma and X-ray emission seen from these structures. However, theoreticians did not fully understand how the acceleration process would work.
Frederico Fiuza of the SLAC National Accelerator Laboratory in Menlo Park, California, and his team decided to scale their simulations of an AGN jet to an unprecedented scale: while previous simulations would have modeled jet plasma as a fluid and give a rough description, they would model at the level of the individual particles. The high computational power required for this project has prevented others from trying before, explains SLAC team member Paulo Alves.
The simulations of the team performed on a supercomputer followed 550 billion particles in a frame of reference that "moves" particles flowed out of a smooth area of the beam outward through a kink instability where the field constantly wobbled and bent , The researchers did not try to follow the particles as they moved through the entire jet, but focused on activity near instability.
The team found that the continuous shaking of the magnetic field induces a strong electric field, which is generally perpendicular to the jet of the smoother parts of the magnetic field that extend around the jet. Normally, however, this electric field can not accelerate charged particles because they normally can only move along magnetic field lines. However, in the simulations, the kink instability confused the magnetic field lines so that the particles could traverse them and be brought by the electric field to energies high enough to become UHECRs.
"What's so special about our recent work," says Fiuza, "shows that a mechanism that works naturally in the magnetic field structure known in AGN jets can simultaneously explain the generation of high-energy radiation and high-energy cosmic radiation , "
" I think this document represents a major step forward in understanding the transformation of magnetic energy into energetic particles in jets, "says plasma physicist James Drake of the University of Maryland, College Park. Although the simulations generate strong electric fields, Drake was not convinced that protons could be accelerated until