Radio Galaxies - The TeV Challenge

Abstract

Over the past decade, our knowledge of the γ-ray sky has been revolutionized by ground-and space-based observatories by detecting photons up to several hundreds of tera-electron volt (TeV) energies. A major population of the γ-ray bright objects are active galactic nuclei (AGN) with their relativistic jets pointed along our line-of-sight. Gamma-ray emission is also detected from nearby mis-aligned AGN such as radio galaxies. While the TeV-detected radio galaxies (T eV Rad) only form a small fraction of the γ-ray detected AGN, their multi-wavelength study offers a unique opportunity to probe and pinpoint the high-energy emission processes and sites. Even in the absence of substantial Doppler beaming T eV Rad are extremely bright objects in the TeV sky (luminosities detected up to 10 45 erg s −1 ), and exhibit flux variations on timescales shorter than the event-horizon scales (flux doubling timescale less than 5 minutes). Thanks to the recent advancement in the imaging capabilities of high-resolution radio interferometry (millimeter very long baseline interferometry, mm-VLBI), one can probe the scales down to less than 10 gravitational radii in T eV Rad, making it possible not only to test jet launching models but also to pinpoint the high-energy emission sites and to unravel the emission mechanisms. This review provides an overview of the high-energy observations of T eV Rad with a focus on the emitting sites and radiation processes. Some recent approaches in simulations are also sketched. Observations by the near-future facilities like Cherenkov Telescope Array, short millimeter-VLBI, and high-energy polarimetry instruments will be crucial for discriminating the competing high-energy emission models.

Very High-Energy Emission from the Direct Vicinity of Rapidly Rotating Black Holes

“…Thus, we can predict an anti-correlation between the ADAF-emitted infrared (IR)/optical and the gap-emitted HE/VHE fluxes for stellar-mass BHs, and between the ADAF-emitted radio and the gap-emitted VHE fluxes for supermassive BHs. This forms a contrast to the standard shock-in-jet scenario, whose natural prediction will be a correlation between the radio/IR/optical and the HE/VHE fluxes (for reviews or catalogues of gamma-ray observations of AGNs, see e.g., [126,[141][142][143][144][145]). This anti-correlation or correlation can be detectable both for stellar-mass and supermassive BHs.…”