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.

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

Context. The Galactic Plane Scan (GPS) was one of the core observation programmes of the INTEGRAL satellite. The highly variable accreting pulsar OAO 1657−415 was frequently observed within the GPS. Aims. We investigate the spectral and timing properties of OAO 1657−415 and their variability on short and long time scales in the energy range 6−160 keV. Methods. Using standard extraction tools and custom software for extracting INTEGRAL data we analysed energy-resolved light curves with a time resolution of one second -mainly data of the ISGRI instrument. We also analysed phase-averaged broad band spectra -including JEM-X spectra -and pulse-phase resolved spectra of ISGRI. Results. During the time covered by the INTEGRAL observations, the pulse period evolution shows an initial spin-down, which is followed by an equally strong spin-up. In combining our results with historical pulse period measurements (correcting them for orbital variation) and with stretches of continuous observations by BATSE, we find that the long-term period evolution is characterised by a long-term spin-up overlayed by sets of relative spin-down/spin-up episodes, which appear to repeat quasi-periodically on a 4.8 yr time scale. We measure an updated local ephemeris and confirm the previously determined orbital period with an improved accuracy. The spectra clearly change with pulse phase. The spectrum measured during the main peak of the pulse profile is particularly hard. We do not find any evidence of a cyclotron line, wether in the phase-averaged spectrum or in phase-resolved spectra.

We present the first intermediate-resolution (l/Dl ϭ 3000) spectrum of the bright quasi-stellar object 3C 273 at wavelengths between 900 and 1200 Å . Observations were performed with the Berkeley spectrograph aboard the ORFEUS II mission. We detect Lyb counterparts to intergalactic Lya features identified by Morris and coworkers at cz ϭ 19,900, 1600, and 1000 km s ; counterparts to other putative Lya clouds along the sight Ϫ1 line are below our detection limit. The strengths of the two very low redshift Lyb features, which are believed to arise in Virgo intracluster gas, exceed preflight expectations (Weymann and coworkers), suggesting that the previous determination of the cloud parameters may underestimate the true column densities. A curve-of-growth analysis sets a minimum H i column density of 4 # 10 14 cm for the 1600 km s cloud. We find marginally Ϫ2 Ϫ1 significant evidence for Galactic H along the sight line, with a total column density of about 10 15 cm . We Ϫ2 2 detect the stronger interstellar O vi doublet member unambiguously; the weaker member is blended with other features. If the Doppler b-value for O vi is comparable to that determined for N v by Sembach and collaborators, then the O vi column density is (7 ‫ע‬ 2) # 10 14 cm , significantly above the only previous estimate, by Davidsen. Ϫ2The O vi/N v ratio is about 10, consistent with the low end of the range observed in the disk, as shown in the compilation by Hurwitz & Bowyer. Additional interstellar species detected for the first time toward 3C 273 (at modest statistical significance) include P ii, Fe iii, Ar i, and S iii.