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.

Abstract. During the second flight of the ORFEUS-SPAS mission in November/December 1996, the Echelle spectrometer was used extensively by the Principal and Guest Investigator teams as one of the two focal plane instruments of the ORFEUS telescope. We present the inflight performance and the principles of the data reduction for this instrument. The wavelength range is 90 nm to 140 nm, the spectral resolution is significantly better than λ/∆λ = 10 000, where ∆λ is measured as FWHM of the instrumental profile. The effective area peaks at 1.3 cm 2 near 110 nm. The background is dominated by straylight from the Echelle grating and is about 15% in an extracted spectrum for spectra with a rather flat continuum. The internal accuracy of the wavelength calibration is better than ± 0.005 nm.

We present results on novel image intensifier tubes for single photon detection. We have adopted an image charge coupling technique that allows a read-out of image intensifiers with good imaging properties and much superior time resolution than obtainable with the standard phosphor screen read-out. Although combinations of sealed microchannel plate detector tubes with position and time sensitive anode structures have already been reported, our method has the advantage that the superficial electrode array has not to be implemented inside the tube. We couple the image charge from a highresistive anode layer through the vacuum housing to a wedge-and-strip or delay-line pattern that can be attached from outside. We show results on single photon imaging with special intensifiers produced by Proxitronic GmbH in the visible and UV for an active diameter of 25 mm. The variability of the system, especially a version with a solar-blind UV-cathode and 40 mm active diameter, should open great opportunities for detection tasks in various fields like astronomy, reconnaissance, bioluminescence, atomic physics, and material research, particularly when both good imaging and timing performance are required.

Fast-Neutron imaging with PCII V. Dangendorf A new imaging method that combines high-efficiency fast-neutron detection with sub-ns time resolution is presented. This is achieved by exploiting the high neutron detection efficiency of a thick scintillator and the fast timing capability and flexibility of light-pulse detection with a dedicated image intensifier. The neutron converter is a plastic scintillator slab or, alternatively, a scintillating fibre screen. The scintillator is optically coupled to a pulse counting image intensifier which measures the 2-dimensional position coordinates and the Time-Of-Flight (TOF) of each detected neutron with an intrinsic time resolution of less than 1 ns. Large-area imaging devices with high count rate capability can be obtained by lateral segmentation of the optical readout channels.

The World Space Observatory Ultraviolet (WSO-UV) is a multi-national project lead by the Russian Federal Space Agency (Roscosmos) with the objective of high performance observations in the ultraviolet range. The 1.7 m WSO-UV telescope is equipped with UV spectrographs (responsibility of Russia and Germany) and UV imagers (responsibility of Spain). The UV spectroscopic instrumentation comprises two high resolution echelle spectrographs operating in wavelength ranges of 102-176 nm and 174-310 nm respectively, and a Long Slit Spectrograph designed to operate in the range of 102-310 nm. All three spectrographs represent individual instruments. In order to save mass while maintaining high stiffness, the instruments are combined to a monoblock, World Space Observatory Ultraviolet Spectrographs (WUVS). Due to strict technical requirements stated by the customer the material CeSiC (provided by the company ECM) has been selected for the design of the spectrograph structure. In contrast to aluminium, the stable structure of CeSiC is significantly less sensitive to thermal gradients. No further mechanism for focus correction with high functional, technical and operational complexity and corresponding additional System costs are necessary. Using CeSiC also relaxes the thermal control requirements of ±5°C, which represents a considerable cost driver for the S/C design. The phase B2 study of the WUVS instrument finished in December 2010 in collaboration with Russia and with industrial support of the Kayser-Threde company. It included construction of a Structural Thermal Model (STM) for verification of thermal and mechanical loads, stability with respect to thermal distortions and CeSiC manufacturing feasibility.