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

In the present study, considering the physical conditions that are relevant in interactions between supernova remnants (SNRs) and dense molecular clouds for triggering star formation we have built a diagram of SNR radius versus cloud density in which the constraints above delineate a shaded zone where star formation is allowed. We have also performed fully 3D radiatively cooling numerical simulations of the impact between SNRs and clouds under different initial conditions in order to follow the initial steps of these interactions. We determine the conditions that may lead either to cloud collapse and star formation or to complete cloud destruction and find that the numerical results are consistent with those of the SNR–cloud density diagram. Finally, we have applied the results above to the β Pictoris stellar association which is composed of low‐mass post‐T Tauri stars with an age of 11 Myr. It has been recently suggested that its formation could have been triggered by the shock wave produced by an SN explosion localized at a distance of about 62 pc that may have occurred either in the Lower Centaurus Crux or in the Upper Centaurus Lupus which are both nearby older subgroups of that association (Ortega and co‐workers). Using the results of the analysis above we have shown that the suggested origin for the young association at the proposed distance is plausible only for a very restricted range of initial conditions for the parent molecular cloud, that is, a cloud with a radius of the order of 10 pc and density of the order of 20 cm−3 and a temperature of the order of 50–100 K.

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Turbulence is ubiquitous in astrophysics. It radically changes many astrophysical phenomena, in particular, the propagation and acceleration of cosmic rays. We present the modern understanding of compressible magnetohydrodynamic (MHD) turbulence, in particular its decomposition into Alfvén, slow and fast modes, discuss the density structure of turbulent subsonic and supersonic media, as well as other relevant regimes of astrophysical turbulence. All this information is essential for understanding the energetic par-A. Lazarian

We report the reemergence of a new broadband emission through a detailed and systematic study of the multiwavelength spectral and temporal behavior of OJ 287 after its first-ever reported very high energy activity in 2017 to date, which includes the second-highest X-ray flux of the source. The source shows high optical to X-ray flux variations, accompanied mainly by strong spectral changes. The optical to X-ray flux variations are correlated and simultaneous except for two durations when they are anticorrelated. The flux variations, however, are anticorrelated with the X-ray spectral state while correlated with optical–UV (ultraviolet). Weekly binned Fermi-LAT data around the duration of the highest X-ray activity show a few detections with a log-parabola model but none with a power law, yet the extracted LAT spectral energy distribution of the high-activity duration for both the models is similar and shows a hardening above 1 GeV. Further, near-infrared data indicate strong spectral change, resembling a thermal component. Overall, the combined optical to gamma-ray broadband spectrum establishes the observed variations to a new high-energy-peaked broadband emission component, similar to the one seen during the highest reported X-ray flux state of the source in 2017. The observed activities indicate some peculiar features that seem to be characteristic of this emission component, while its appearance a few years around the claimed ∼12 yr optical outbursts strongly indicates a connection between the two.