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 physical mechanism responsible for the short outbursts in a recently recognised class of high mass X-ray binaries, the supergiant fast X-ray transients (SFXTs), is still unknown. Two main hypotheses have been proposed to date: the sudden accretion by the compact object of small ejections originating in a clumpy wind from the supergiant donor, or outbursts produced at (or near) the periastron passage in wide and eccentric orbits, to explain the low (∼10 32 erg s −1 ) quiescent emission. Neither proposed mechanisms explain the whole phenomenology of these sources. IGR J11215-5952, discovered in April 2005 by the INTEGRAL satellite, is a SFXT which undergoes an outburst every 329 days, a periodicity likely associated with the orbital period of the binary system. Aims. We propose a new explanation for the outburst mechanism, based on the X-ray observations of the unique SFXT known to display periodic outbursts, IGR J11215-5952. Methods. We performed three Target of Opportunity (ToO) Results. XMM-Newton observed the source on 2007 February 9, for 23 ks, at the peak of the outburst, while INT EGRAL started the observation two days later, failing to detect the source, which had already undergone the decaying phase of the fast outburst. XMM-Newton data show large variability, with a bright flare at the beginning of the observation (lasting about 1 h), followed by a lower intensity phase (about one order of magnitude fainter) with a large variability as well as low level flaring activity. The spin periodicity discovered by RXTE is confirmed, and a spin-phase spectral variability is observed and studied in detail. The Swift campaign performed in July 2007 reveals a second outburst on 2007 July 24, as bright as that observed about 165 days before. Conclusions. The new X-ray observations allow us to propose an alternative hypothesis for the outburst mechanism in SFXTs, linked to the possible presence of a second wind component, in the form of an equatorial disc from the supergiant donor. We discuss the applicability of the model to the short outburst durations of all other SFXTs, where a clear periodicity in the outbursts has not been found yet. The new outburst from IGR J11215-5952 observed in July suggests that the true orbital period is ∼165 days, instead of 329 days, as previously thought.

Using the Chandra Advanced CCD Imaging Spectrometer Imaging array (ACIS-I), we have carried out a deep hard X-ray observation of the Galactic plane region at (l, b) ≈ (28. • 5, 0. • 0), where no discrete X-ray source had been reported previously. We have detected 274 new point X-ray sources (4 σ confidence) as well as strong Galactic diffuse emission within two partially overlapping ACIS-I fields (∼ 250 arcmin 2 in total). The point source sensitivity was 1 code 662, NASA/GSFC, ∼ 3 × 10 −15 erg s −1 cm −2 in the hard X-ray band (2 -10 keV) and ∼ 2 × 10 −16 erg s −1 cm −2 in the soft band (0.5 -2 keV). Sum of all the detected point source fluxes accounts for only ∼ 10 % of the total X-ray flux in the field of view. Even hypothesizing a new population of much dimmer and numerous Galactic point sources, the total observed X-ray flux cannot be explained. Therefore, we conclude that X-ray emission from the Galactic plane has truly diffuse origin. Removing point sources brighter than ∼ 3 × 10 −15 erg s −1 cm −2 (2-10 keV), we have determined the Galactic diffuse X-ray flux as 6.5 ×10 −11 erg s −1 cm −2 deg −2 (2-10 keV). Only 26 point sources were detected both in the soft and hard bands, indicating that there are two distinct classes of the X-ray sources distinguished by the spectral hardness ratio. Surface number density of the hard sources is only slightly higher than that measured at the high Galactic latitude regions, indicating that majority of the hard sources are background AGNs. Following up the Chandra observation, we have performed a near-infrared (NIR) survey with SOFI at ESO/NTT. Almost all the soft X-ray sources have been identified in NIR and their spectral types are consistent with main-sequence stars, suggesting most of them are nearby X-ray active stars. On the other hand, only 22 % of the hard sources had NIR counterparts, which are presumably Galactic. From X-ray and NIR spectral study, they are most likely to be quiescent cataclysmic variables. Our observation suggests a population of 10 4 cataclysmic variables in the entire Galactic plane fainter than ∼ 2 × 10 33 erg s −1 . We have carried out a precise spectral study of the Galactic diffuse X-ray emission excluding the point sources. Confirming previous results, we have detected prominent emission lines from highly ionized heavy elements in the diffuse emission. In particular, central energy of the iron emission line was determined as 6.52 +0.08 −0.14 keV (90 % confidence), which is significantly lower than what is expected from a plasma in thermal equilibrium. The downward shift of the iron line center energy suggests non-equilibrium ionization states of the plasma, or presence of the non-thermal process to produce 6.4 keV fluorescent lines.

We have developed a stellar wind model for OB supergiants to investigate the effects of accretion from a clumpy wind on the luminosity and variability properties of high‐mass X‐ray binaries. Assuming that the clumps are confined by ram pressure of the ambient gas and exploring different distributions for their mass and radii, we computed the expected X‐ray light curves in the framework of the Bondi–Hoyle accretion theory, modified to take into account the presence of clumps. The resulting variability properties are found to depend not only on the assumed orbital parameters but also on the wind characteristics. We have then applied this model to reproduce the X‐ray light curves of three representative high‐mass X‐ray binaries: two persistent supergiant systems (Vela X−1 and 4U 1700−377) and the supergiant fast X‐ray transient IGR J11215−5952. The model can reproduce the observed light curves well, but requiring in all cases an overall mass loss from the supergiant about a factor of 3–10 smaller than the values inferred from ultraviolet lines studies that assume a homogeneous wind.

THESEUS is a space mission concept aimed at exploiting Gamma-Ray Bursts for investigating the early Universe and at providing a substantial advancement of multi-messenger and time-domain astrophysics. These goals will be achieved through a unique combination of instruments allowing GRB and X-ray transient detection over a broad field of view (more than 1sr) with 0.5-1 arcmin localization, an energy band extending from several MeV down to 0.3 keV and high sensitivity to transient sources in the soft X-ray domain, as well as on-board prompt (few minutes) followup with a 0.7 m class IR telescope with both imaging and spectroscopic capabilities. THESEUS will be perfectly suited for addressing the main open issues in cosmology such as, e.g., star formation rate and metallicity evolution of the inter-stellar and intra-galactic medium up to redshift ∼10, signatures of Pop III stars, sources and physics of reionization, and the faint end of the galaxy luminosity function. In addition, it will provide unprecedented capability to monitor the X-ray variable sky, thus detecting, localizing, and identifying the electromagnetic counterparts to sources of gravitational radiation, which may be routinely detected in the late '20s / early '30s by next generation facilities like aLIGO/ aVirgo, eLISA, KAGRA, and Einstein Telescope. THESEUS will also provide powerful synergies with the next generation of multi-wavelength observatories (e.g., LSST, ELT, SKA, CTA, ATHENA).