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

We study the feedback between heating and cooling of the intra-cluster medium (ICM) in cooling flow (CF) galaxies and clusters. We adopt the popular view that the heating is due to an active galactic nucleus (AGN), i.e. a central black hole accreting mass and launching jets and/or winds. We propose that the feedback occurs with the entire cool inner region (r <~ 5-30 kpc), where the non-linear over-dense blobs of gas with a density contrast >~2 cool fast and are removed from the ICM before experiencing the next major AGN heating event. We term this scenario "cold-feedback". Some of these blobs cool and sink toward the central black hole, while others might form stars and cold molecular clouds. We derive the conditions under which the dense blobs formed by perturbations might cool to low temperatures (T <~ 10^4 K), and feed the black hole. The main conditions are found to be: (1) An over-dense blob must be prevented from reaching an equilibrium position in the ICM: therefore it has to cool fast, and the density profile of the ambient gas should be shallow; (2) Non-linear perturbations are required: they might have chiefly formed by previous AGN activity; (3) The cooling time of these non-linear perturbations should be short relative to few times the typical interval between successive AGN outbursts. (4) The blobs should be magnetically disconnected from their surroundings, in order not to be evaporated by thermal conduction.Comment: Replaced wiht the version accepted by the Ap

Employing XMM-Newton EPIC data we perform a detailed comparison between different spectral models to test whether the gas in cooling-flows is multi-phase or not. Our findings all point in the same direction, namely that gas in cooling-flows does not show the wide distribution of temperatures expected from standard multi-phase models. This result has profound implications for cooling-flow models. Firstly, the large absorption column depths inferred by previous analysis of cooling-flow spectra are most likely an artifact following from the application of an incorrect spectral model to the data. Secondly, the mass deposition and mass flow are likely to be much smaller than previously thought. Last, but perhaps not least, the term "cooling-flow" cluster is probably no longer appropriate, as it describes a phenomenon of smaller entity and impact than previously thought. We propose to substitute it with that of "cool-core" cluster. The latter definition is less ambitious than the first, as it reflects only an observational fact rather than an inferred physical property, the flow, but has the undeniable advantage of being firmer.