Black Hole as Extreme Particle Accelerator

Astrophysicists have theorized for decades that the mysterious, ultrahigh-energy cosmic rays (UHECRs) detected on Earth could come from active galactic nuclei (AGNs), the supermassive black holes at the centers of galaxies. The particle acceleration mechanism, however, has been unclear. Now a team of researchers has used simulations of unprecedented size to propose a mechanism. The results strengthen the idea of AGNs as UHECR sources, although some experts caution 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 108eV. UHECRs have energies above 1018eV and have been sporadically detected on Earth since the 1960s, though their sources have remained uncertain. A recent coincident detection of a high-energy neutrino and a gamma ray from an AGN was seen as strong evidence for AGNs as UHECR sources, since both of these events can be explained by the acceleration of protons to ultrahigh energies [1]. Still, other UHECR-source candidates, such as relativistic supernovae and gamma-ray bursts, remain viable possibilities.

Astrophysical jets are beams of ionized matter emitted in both directions along the axis of rotation of a massive astronomical object, such as an AGN. Relativistic AGN jets—where some particles move at close to the speed of light—have been thoroughly studied, both observationally and theoretically. Most of their energy is stored in powerful magnetic fields that take on a helical structure. Previous simulations have demonstrated that the jet plasma is not stable; interactions with the interstellar medium cause distortions of the jet called kink instabilities, where the jet’s column-like shape bends and twists.

[Read More]