Very high-energy gamma rays from ultra-fast outflows

Context. Ultra-fast outflows (UFOs) from active galactic nuclei (AGNs) are expected to lead to the formation of sub-relativistic strong shocks expanding in a dense, circumnuclear medium, and thus have the potential to be efficient particle accelerators and proficient sources of gamma rays and neutrinos. Aims. We aim to investigate the detectability of a sample of nearby identified UFOs in gamma rays and neutrinos with current and next-generation instruments. Methods. We modeled the acceleration of particles at the strong shocks of UFOs and estimated the associated gamma-ray and neutrino signal. We adopted our model to investigate the prospects for detection with current and next-generation observatories. Results. We find that several UFOs could be detectable in the very-high-energy (VHE) domain – for example, by the Cherenkov Telescope Array Observatory (CTAO) – even if they remain undetected by Fermi -LAT in the high-energy range. Detectability is favored for hard proton spectra (spectral index α ≲ 3.9), high acceleration efficiencies, and amplified magnetic fields. Our results suggest that next-generation VHE observatories could detect the first gamma-ray signatures of AGN UFOs, providing a new probe of particle acceleration in sub-relativistic shocks.