Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5–10 improvement in sensitivity in the 100 GeV–10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA.

A search for a very-high-energy (VHE; $\ge 100\mathrm{GeV}$) $\gamma$-ray signal from self-annihilating particle dark matter (DM) is performed towards a region of projected distance $r\sim 45–150\mathrm{pc}$ from the Galactic center. The background-subtracted $\gamma$-ray spectrum measured with the High Energy Stereoscopic System (H.E.S.S.) $\gamma$-ray instrument in the energy range between 300 GeV and 30 TeV shows no hint of a residual $\gamma$-ray flux. Assuming conventional Navarro-Frenk-White and Einasto density profiles, limits are derived on the velocity-weighted annihilation cross section $⟨\sigma v⟩$ as a function of the DM particle mass. These are among the best reported so far for this energy range and in particular differ only little between the chosen density profile parametrizations. In particular, for the DM particle mass of $\sim 1\mathrm{TeV}$, values for $⟨\sigma v⟩$ above $3\times {10}^{-25}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ are excluded for the Einasto density profile.

Gamma-ray line signatures can be expected in the very-high-energy ($E_{\gamma }>100\mathrm{GeV}$) domain due to self-annihilation or decay of dark matter (DM) particles in space. Such a signal would be readily distinguishable from astrophysical $\gamma$-ray sources that in most cases produce continuous spectra that span over several orders of magnitude in energy. Using data collected with the H.E.S.S. $\gamma$-ray instrument, upper limits on linelike emission are obtained in the energy range between $\sim 500\mathrm{GeV}$ and $\sim 25\mathrm{TeV}$ for the central part of the Milky Way halo and for extragalactic observations, complementing recent limits obtained with the Fermi-LAT instrument at lower energies. No statistically significant signal could be found. For monochromatic $\gamma$-ray line emission, flux limits of $(2\times {10}^{-7}–2\times {10}^{-5}){\mathrm{m}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$ and $(1\times {10}^{-8}–2\times {10}^{-6}){\mathrm{m}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$ are obtained for the central part of the Milky Way halo and extragalactic observations, respectively. For a DM particle mass of 1 TeV, limits on the velocity-averaged DM annihilation cross section $⟨\sigma v{⟩}_{\chi \chi \to \gamma \gamma }$ reach $\sim {10}^{-27}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$, based on the Einasto parametrization of the Galactic DM halo density profile.

Current Cherenkov telescopes have identified a population of ultra-high-frequency peaked BL Lac objects (UHBLs), also known as extreme blazars, that exhibit exceptionally hard TeV spectra, including 1ES 0229+200, 1ES 0347−121, RGB J0710+591, 1ES 1101−232, and 1ES 1218+304. Although one-zone synchrotron-self-Compton (SSC) models have been generally successful in interpreting the high-energy emission observed in other BL Lac objects, they are problematic for UHBLs, necessitating very large Doppler factors and/or extremely high minimum Lorentz factors of the emitting leptonic population. In this context, we have investigated alternative scenarios where hadronic emission processes are important, using a newly developed (lepto-)hadronic numerical code to systematically explore the physical parameters of the emission region that reproduces the observed spectra while avoiding the extreme values encountered in pure SSC models. Assuming a fixed Doppler factor δ = 30, two principal parameter regimes are identified, where the high-energy emission is due to: (1) proton-synchrotron radiation, with magnetic fields B ∼ 1–100 G and maximum proton energies E_{p; max} ≲ 10¹⁹ eV; and (2) synchrotron emission from p–γ-induced cascades as well as SSC emission from primary leptons, with B ∼ 0.1–1 G and E_{p; max} ≲ 10¹⁷ eV. This can be realized with plausible, sub-Eddington values for the total (kinetic plus magnetic) power of the emitting plasma, in contrast to hadronic interpretations for other blazar classes that often warrant highly super-Eddington values.