Magnetic reconnection maximized by turbulence explains formation of ultra-high energy cosmic rays

A study conducted at the University of São Paulo simulated the phenomenon and was highlighted by Nature Astronomy.

By José Tadeu Arantes  |  Agência FAPESP – Cosmic rays are hitting Earth all the time. They come from the Sun, other parts of the Milky Way or distant galaxies. Cosmic rays are also formed within Earth’s atmosphere by interaction between those that come from afar and local matter. For less energetic cosmic rays, up to 10¹¹ electron-volts (eV), the frequency is one per square meter per second, but there are cosmic rays with extremely high levels of energy, which can reach 10²⁰ eV. These are rare, appearing with a frequency of one per square kilometer per century. For a rough feeling of what 10²⁰ eV means, it may help to recall that the Large Hadron Collider (LHC), the world’s largest particle accelerator, can reach particle energy levels of at most 10¹³ eV. Extremely high-energy cosmic rays are up to 10 million times more energetic.

The standard model to explain low-energy particle acceleration is based on shock waves. The particles are propelled by wavefronts, collide many times, and accelerate more and more. However, this model does not seem to work for ultra-high energy particles. An alternative explanation is required in these cases. In an endeavor to work out such a theory, researchers at the University of São Paulo (USP) in Brazil conducted a study on particle acceleration in relativistic jets from active galactic nuclei, publishing an article about their work in The Astrophysical Journal entitled “Particle acceleration by magnetic reconnection in relativistic jets: The transition from small to large scales”. The article was so well-received that it was picked out for the “Research Highlights” section of Nature Astronomy.

The study was conducted at the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG-USP) by Tania Medina Torrejón and Elisabete de Gouveia Dal Pino, her PhD thesis advisor at the time, with collaboration by Grzegorz Kowal, a professor at the School of Arts, Sciences and Humanities (EACH-USP). It was funded by FAPESP via four projects (13/10559-5, 19/03301-8, 21/06502-4 and 09/54006-4).

“We discovered in a computer simulation that particle acceleration in jets from active galactic nuclei is caused by magnetic reconnection and maximized by the effects of turbulence,” Gouveia Dal Pino told Agência FAPESP.

Owing to the rotation of the accretion disk formed by matter collapsing in the black hole at the center of the galaxy and also to the spin of the active nucleus, jets rebounding from the black hole in both directions orthogonally to the disk have helical magnetic fields. The study considered a portion of a jet in the vicinity of the black hole and injected test particles into it.

Because field lines with opposite polarities attract each other, this produced incessant reconfiguration of the field. Particle acceleration at the sites of magnetic reconnection was driven mainly by “Fermi acceleration”, analogously to what happens in acceleration by shock waves.

“This explanation was first put forward in 2005 by myself and Alex Lazarian, a researcher at the University of Wisconsin [in the U.S.]. In this process, particles trapped between the lines of the magnetic field undergo reconnection, colliding several times with magnetic fluctuations. This leads to acceleration of the particles and exponential growth of their energy. In the jet, turbulence inside the flow distorts the lines of force and creates points of ever-faster reconnection. Particles accelerated in these reconnection regions via field lines reach velocities very close to the speed of light,” Gouveia Dal Pino said.

Acceleration by reconnection may explain the more energetic phenomena observed in jets from active galactic nuclei, such as the emission of gamma rays and neutrinos.

The article “Particle acceleration by magnetic reconnection in relativistic jets: the transition from small to large scales” is at: iopscience.iop.org/article/10.3847/1538-4357/acd699.