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 Burst (GRB) prompt emission spectra in the keV-MeV energy range are usually considered as adequately fitted with the empirical Band function. Recent observations with the Fermi Gamma-Ray Space Telescope (Fermi) revealed deviations from the Band function, sometimes in the form of an additional black-body (BB) component, while on other occasions in the form of an additional power law (PL) component extending to high energies. In this article we investigate the possibility that the three components may be present simultaneously in the prompt emission spectra of two very bright GRBs (080916C and 090926A) observed with Fermi, and how the three components may affect the overall shape of the spectra. While the two GRBs are very different when fitted with a single Band function, they look like "twins" in the three-component scenario. Through fine-time spectroscopy down to the 100 ms time scale, we follow the evolution of the various components. We succeed in reducing the number of free parameters in the three-component model, which results in a new semi-empirical model-but with physical motivations-to be competitive with the Band function in terms of number of degrees of freedom. From this analysis using multiple components, the Band function is globally the most intense component, although the additional PL can overpower the others in sharp time structures. The Band function and the BB component are the most intense at early times and globally fade across the burst duration. The additional PL is the most intense component at late time and may be correlated with the extended high-energy emission observed thousands of seconds after the burst with Fermi/Large Area Telescope (LAT). Unexpectedly, this analysis also shows that the additional PL may be present from the very beginning of the burst, where it may even overpower the other components at low energy. We investigate the effect of the three components on the new time-resolved luminosity-hardness relation in both the observer and rest frames and show that a strong correlation exists between the flux of the non-thermal Band function and its E peak only when the three components are fitted simultaneously to the data (i.e., F NT i -E NT peak,i relation). In addition, this result points toward a universal relation between those two quantities when transposed to the central engine rest frame for all GRBs (i.e., L NT i -E rest,NT peak,i relation). We discuss theoretical implications of the three spectral components within this new empirical model. The BB component can be interpreted as the photosphere emission of a magnetized relativistic outflow. The Band component can be interpreted as synchrotron radiation in an optically thin region above the photosphere, either from internal shocks or magnetic field dissipation. The extra power law component extending to high energies likely has an inverse Compton origin of some sort, even though its extension to a much lower energy remains a mystery.

We present the fourth in a series of catalogs of gamma-ray bursts (GRBs) observed with Fermi 's Gamma-Ray Burst Monitor (Fermi -GBM). It extends the six year catalog by four more years, now covering the 10 year time period from trigger enabling on 2008 July 12 to 2018 July 11. During this time period GBM triggered almost twice a day on transient events of which we identified 2356 as cosmic GRBs. Additional trigger events were due to solar flare events, magnetar burst activities, and terrestrial gamma-ray flashes. The intention of the GBM GRB catalog series is to provide updated information to the community on the most important observables of the GBM-detected GRBs. For each GRB the location and main characteristics of the prompt emission, the duration, peak flux, and fluence are derived. The latter two quantities are calculated for the 50-300 keV energy band, where the maximum energy release of GRBs in the instrument reference system is observed and also for a broader energy band from 10-1000 keV, exploiting the full energy range of GBM's low-energy detectors. Furthermore, information is given on the settings of the triggering criteria and exceptional operational conditions during years seven to ten in the mission. This fourth catalog is an official product of the Fermi -GBM science team, and the data files containing the complete results are available from the

Since its launch in 2008, the Fermi Gamma-ray Burst Monitor (GBM) has triggered and located on average approximately two γ-ray bursts (GRBs) every three days. Here, we present the third of a series of catalogs of GRBs detected by GBM, extending the second catalog by two more years through the middle of 2014 July. The resulting list includes 1405 triggers identified as GRBs. The intention of the GBM GRB catalog is to provide information to the community on the most important observables of the GBM-detected GRBs. For each GRB, the location and main characteristics of the prompt emission, the duration, peak flux, and fluence are derived. The latter two quantities are calculated for the 50–300 keV energy band where the maximum energy release of GRBs in the instrument reference system is observed, and also for a broader energy band from 10 to 1000 keV, exploiting the full energy range of GBM's low-energy [Nai[Tl)] detectors. Using statistical methods to assess clustering, we find that the hardness and duration of GRBs are better fit by a two-component model with short-hard and long-soft bursts than by a model with three components. Furthermore, information is provided on the settings and modifications of the triggering criteria and exceptional operational conditions during years five and six in the mission. This third catalog is an official product of the Fermi GBM science team, and the data files containing the complete results are available from the High-Energy Astrophysics Science Archive Research Center.