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

The population of high‐energy and very high‐energy γ‐ray sources, detected with EGRET and the new generation of ground‐based Cherenkov telescopes, forms a reduced but physically important sample. Most of these sources are extragalactic (e.g. blazars), while among the galactic ones there are pulsars and supernova remnants. The microquasar LS 5039, previously proposed to be associated with an EGRET source by Paredes et al., has recently been detected at TeV energies, confirming that microquasars should be regarded as a class of high‐energy γ‐ray sources. To model and understand how the energetic photons are produced and escape from LS 5039, it is crucial to unveil the nature of the compact object, which remains unknown. Here, we present new intermediate‐dispersion spectroscopy of this source, which, combined with values reported in the literature, provides an orbital period of Porb= 3.906 03 ± 0.000 17 d, a mass function f(M) = 0.0053 ± 0.0009 M⊙ and an eccentricity e= 0.35 ± 0.04. Atmosphere model fitting to the spectrum of the optical companion, together with our new distance estimate of d= 2.5 ± 0.1 kpc, yields RO= 9.3+0.7−0.6 R⊙, log (LO/ L⊙) = 5.26 ± 0.06 and MO= 22.9+3.4−2.9 M⊙. These, combined with our dynamical solution and the assumption of pseudo‐synchronization, yield an inclination and a compact object mass MX= 3.7+1.3−1.0 M⊙. This is above neutron star masses for most of the standard equations of state and, therefore, we propose that the compact object in LS 5039 is a black hole. We finally discuss the implications of our orbital solution and new parameters of the binary system on the CNO products, the accretion/ejection energetic balance, the supernova explosion scenario and the behaviour of the very high‐energy γ‐ray emission with the new orbital period.

New optical spectroscopy of the high‐mass X‐ray binary microquasar LS I +61 303 is presented. Eccentric orbital fits to our radial velocity measurements yield updated orbital parameters in good agreement with previous work. Our orbital solution indicates that the periastron passage occurs at radio phase 0.23 and the X‐ray/radio outbursts are triggered 2.5–4 d after the compact star passage. The spectrum of the optical star is consistent with a B0 V spectral type and contributes ∼65 per cent of the total light, the remainder being the result of emission by a circumstellar disc. We also measure the projected rotational velocity to be v sin i≃ 113 km s−1.

-2 -Microquasars are stellar x-ray binaries that behave as a scaled down version of extragalactic quasars. The star LS 5039 is a new microquasar system with apparent persistent ejection of relativistic plasma at a 3 kiloparsec distance from the sun. It may also be associated with a γ-ray source discovered by the Energetic Gamma Ray Experiment Telescope (EGRET) on board the COMPTON-Gamma Ray Observatory satellite. Before the discovery of LS 5039, merely a handful of microquasars had been identified in the Galaxy, and none of them was detected in high-energy γ-rays.-3 -The V = 11.2 magnitude star LS 5039 (1) has been recently identified as a nearby high-mass x-ray binary with spectral type O7V((f)) (2) and persistent radio emission (3,4). Here, we report high-resolution radio observations with the Very Long Baseline Array (VLBA) and the Very Large Array (VLA) that reveal that LS 5039 is resolved into bipolar radio jets emanating from a central core.Because LS 5039 appeared unresolved (≤ 0.1 ′′ ) to the VLA alone, we proceeded to study this object with milliarc sec resolution using the VLBA at the frequency of 5 GHz (6 cm wavelength) on 8 May 1999. The VLA in its phased array mode, equivalent to a dish of 115 m diameter, also participated as an independent station, providing sensitive baselines with the VLBA antennas. The source 3C345 was used as a fringe-finder, whereas J1733−1304 was the phasing source for the VLA. The data were calibrated using standard procedures in unconnected radio interferometry. The resulting pattern of the observed visibility amplitudes, decaying as a function of baseline length, indicated that LS 5039 had structure at milliarc sec scales.The final synthesis map ( Fig. 1) shows that bipolar jets emerge from a central core. A deconvolved angular size of about 2 milliarc sec is estimated for the core. The jets extend over 6 milliarc sec on the sky oriented along a position angle (PA) of 125• with respect to the North, and they account for 20% of the total 16 mJy flux density. To obtain some order of magnitude estimates, we will assume that the overall size of the radio source is approximately 6 × 2 milliarc sec 2 . This implies a high brightness temperature of ∼ 9.4 × 10 7 K, indicating synchrotron radiation. The LS 5039 radio spectrum as a function of frequency ν, namely S ν ∝ ν α , often displays a negative spectral index α = −0.5 in agreement with a non-thermal optically thin emission mechanism (3,4). The detection of jets occurred at a time when the source was at its typical persistent level of radio emission, and only moderately variable, as inferred from concurrent radio monitoring by the Green Bank Interferometer (GBI) (Fig. 2).The absence of any precursor outburst for the radio jets strongly suggests that they are always present and continuously emanating from the core. The flux density ratio between X-ray binaries with collimated radio jets belong to the class of galactic microquasars.The production of jets is almost certainly related to the capture of matter from a normal star by a...

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