UFOs: The Hidden Powerhouses of Cosmic Rays

The universe is full of mysteries, and one of them is the origin of ultra-high-energy cosmic rays. These cosmic rays are incredibly powerful, but where do they come from? One possible source is ultra-fast outflows, or UFOs, from active galactic nuclei. These UFOs are not the kind that people report seeing in the sky. These UFOs are large-scale, mildly-relativistic outflows that move at speeds up to half the speed of light. They are a common feature of active galactic nuclei, or AGN. UFOs have the potential to accelerate cosmic-ray nuclei to extremely high energies. This is a topic that has not been explored much in the past. Scientists have used 3D simulations to study the cosmic-ray spectrum and the maximum energy that can be achieved in these environments. They applied their findings to 86 observed UFOs. The results show that iron nuclei can be accelerated up to about 10^20 eV at the wind-termination shock in some UFOs. However, the escaping flux is strongly reduced due to interactions with intense AGN photon fields. The maximum energy of nuclei escaping a typical UFO is limited by photodisintegration to below about 10^17 eV. This is a process where high-energy photons break apart the nuclei.In the most extreme cases, which make up about 5-10% of UFOs, helium, nitrogen, and iron nuclei can escape with energies exceeding 10^17. 4, 10^17. 8, and 10^18. 4 eV, respectively. Protons and neutrons, whether they are primary cosmic rays or by-products of photodisintegration, escape UFOs with little reduction in energy. Half of the observed UFOs can reach energies exceeding 10^18 eV. This makes UFOs a possible source of the diffuse cosmic-ray flux between the end of the Galactic cosmic rays and the highest-energy extragalactic flux. For a few UFOs in the sample, nuclei can escape without photodisintegration with energy up to 10^19. 8 eV. This happens during low-emission states of the AGN, which would make UFOs an intermittent source of ultra-high-energy cosmic ray nuclei up to the highest observed energies. The role of UFOs as a source of ultra-high-energy cosmic rays is testable with neutrino telescopes. This is due to a substantial accompanying flux of PeV neutrinos.