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.

Previous detections of individual astrophysical sources of neutrinos are limited to the Sun and the supernova 1987A, whereas the origins of the diffuse flux of high-energy cosmic neutrinos remain unidentified. On 22 September 2017, we detected a high-energy neutrino, IceCube-170922A, with an energy of ~290 tera-electron volts. Its arrival direction was consistent with the location of a known γ-ray blazar, TXS 0506+056, observed to be in a flaring state. An extensive multiwavelength campaign followed, ranging from radio frequencies to γ-rays. These observations characterize the variability and energetics of the blazar and include the detection of TXS 0506+056 in very-high-energy γ-rays. This observation of a neutrino in spatial coincidence with a γ-ray-emitting blazar during an active phase suggests that blazars may be a source of high-energy neutrinos.

No abstract

No abstract

The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project. ?? 2013 Elsevier B.V. All rights reserved

Gamma-ray line signatures can be expected in the very-high-energy (E(γ)>100 GeV) domain due to self-annihilation or decay of dark matter (DM) particles in space. Such a signal would be readily distinguishable from astrophysical γ-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. γ-ray instrument, upper limits on linelike emission are obtained in the energy range between ∼ 500 GeV and ∼ 25 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 γ-ray line emission, flux limits of (2 × 10(-7) -2 × 10(-5)) m(-2) s(-1) sr(-1) and (1 × 10(-8) -2 × 10(-6)) m(-2) s(-1)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 ⟨σv⟩(χχ → γγ) reach ∼ 10(-27) cm(3)s(-1), based on the Einasto parametrization of the Galactic DM halo density profile.

The measurement of an excess in the cosmic-ray electron spectrum between 300 and 800 GeV by the ATIC experiment has -together with the PAMELA detection of a rise in the positron fraction up to ≈100 GeV -motivated many interpretations in terms of dark matter scenarios; alternative explanations assume a nearby electron source like a pulsar or supernova remnant. Here we present a measurement of the cosmic-ray electron spectrum with H.E.S.S. starting at 340 GeV. While the overall electron flux measured by H.E.S.S. is consistent with the ATIC data within statistical and systematic errors, the H.E.S.S. data exclude a pronounced peak in the electron spectrum as suggested for interpretation by ATIC. The H.E.S.S. data follow a power-law spectrum with spectral index of 3.0 ± 0.1(stat.) ± 0.3(syst.), which steepens at about 1 TeV.

Energy spectra of particles accelerated by the rst-order Fermi mechanism are investigated at ultrarelativistic shock waves, outside the range of Lorentz factors considered previously. For particle transport near the shock a numerical method involving small amplitude pitch-angle scattering is applied for ows with Lorentz factors from 3 to 243. For large shocks a convergence of derived energy spectral indices up to the value 1 2:2 is observed for all considered turbulence amplitudes and magnetic eld con gurations. Recently the same index was derived for -ray bursts by W axman [Astrophys. J. Lett. 485, L5 (1997)].In currently favored gamma-ray burst (GRB) models optically thin emitting regions move relativistically, with Lorentz factors of order of a few hundreds (cf. a review [1]). The power-law form of the spectrum often observed at high photon energies suggests the existence of nonthermal population of energetic particles. It was also proposed that GRB sources may produce cosmic ray particles with extremely high energies [2]. Thus modeling of burst sources requires a discussion of particle acceleration processes, possibly the Fermi acceleration at ultrarelativistic shock w a v es. [10]) clari ed a number of issues related to shock waves with velocities reaching 0:98c or the Lorentz factor ' 5 , but { to our knowledge { no one has attempted to discuss particle acceleration at shocks moving with ultrarelativistic velocities characterized with large factors > > 1. The main di culty in modelling an acceleration process at shocks with large is the fact that involved particle distributions are extremely anisotropic in shock, with the particle angular distribution opening angles 1 in the upstream plasma rest frame. When transmitted downstream the shock particles have a limited chance to be scattered so e ciently to reach the shock again, but the energy gain of any such \successful" particle can be comparable to its original energy. As pointed out by Bednarz and Ostrowski [8] any realistic model of particle scattering at magnetohydrodynamic turbulence close to the relativistic shock cannot involve large-angle pointlike scattering. The choice is either to integrate exactly particle equations of motion in some \realistic" structure of the perturbed magnetic eld, or to use a small-angle scattering model for particle momentum. With the angular scattering amplitude << 1 and the mean scattering time t not too short [ t T g ( ) 2 , where T g is the particle gyration period], the last model reproduces the pitch-angle di usion process at small amplitude waves. We prefer that approach to the exact integration of equations of motion of a particle because of its relative simplicity. It is also suggested that it can be reasonably used for modeling particle trajectories in turbulent elds with large amplitude, if small t [ T g ( ) 2 ] is involved. Below, a hybrid method involving the small amplitude pitch-angle scattering is applied for a particle transport near the shock for ows with Lorentz factors from 3 to 243.I. Numerical simul...