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Open AccessReview

Radio Galaxies—The TeV Challenge

by Bindu Rani
NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
NPP Fellow.
Galaxies 2019, 7(1), 23; https://doi.org/10.3390/galaxies7010023
Received: 1 November 2018 / Revised: 11 January 2019 / Accepted: 15 January 2019 / Published: 22 January 2019
(This article belongs to the Special Issue Radio Galaxies at TeV Energies)
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 misaligned AGN such as radio galaxies. While the TeV-detected radio galaxies ( T e V R a d ) 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 e V R a d 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 min). 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 e V R a d , 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 e V R a d 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. View Full-Text
Keywords: active galactic nuclei; radio galaxies; gamma-rays; jets active galactic nuclei; radio galaxies; gamma-rays; jets
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Rani, B. Radio Galaxies—The TeV Challenge. Galaxies 2019, 7, 23.

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