Find in Library

Over the past decade, our knowledge of the <inline-formula> <math display="inline"> <semantics> <mi>γ</mi> </semantics> </math> </inline-formula>-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 <inline-formula> <math display="inline"> <semantics> <mi>γ</mi> </semantics> </math> </inline-formula>-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 (<inline-formula> <math display="inline"> <semantics> <mrow> <mi>T</mi> <mi>e</mi> <mi>V</mi> <mi>R</mi> <mi>a</mi> <mi>d</mi> </mrow> </semantics> </math> </inline-formula>) only form a small fraction of the <inline-formula> <math display="inline"> <semantics> <mi>γ</mi> </semantics> </math> </inline-formula>-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 <inline-formula> <math display="inline"> <semantics> <mrow> <mi>T</mi> <mi>e</mi> <mi>V</mi> <mi>R</mi> <mi>a</mi> <mi>d</mi> </mrow> </semantics> </math> </inline-formula> are extremely bright objects in the TeV sky (luminosities detected up to <inline-formula> <math display="inline"> <semantics> <msup> <mn>10</mn> <mn>45</mn> </msup> </semantics> </math> </inline-formula> erg s<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </semantics> </math> </inline-formula>), 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 <inline-formula> <math display="inline"> <semantics> <mrow> <mi>T</mi> <mi>e</mi> <mi>V</mi> <mi>R</mi> <mi>a</mi> <mi>d</mi> </mrow> </semantics> </math> </inline-formula>, 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 <inline-formula> <math display="inline"> <semantics> <mrow> <mi>T</mi> <mi>e</mi> <mi>V</mi> <mi>R</mi> <mi>a</mi> <mi>d</mi> </mrow> </semantics> </math> </inline-formula> 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.