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

3C 397 is a radio and X-ray bright Galactic supernova remnant (SNR) with an unusual rectangular morphology. Our CO observation obtained with the Purple Mountain Observatory at Delingha reveals that the remnant is well confined in a cavity of molecular gas, and embedded at the edge of a molecular cloud (MC) at the local standard of rest systemic velocity of ∼ 32 km s −1 . The cloud has a column density gradient increasing from southeast to northwest, perpendicular to the Galactic plane, in agreement with the elongation direction of the remnant. This systemic velocity places the cloud and SNR 3C 397 at a kinematic distance of ∼ 10.3 kpc. The derived mean molecular density (∼ 10-30 cm −3 ) explains the high volume emission measure of the X-ray emitting gas. A 12 CO line broadening of the ∼ 32 km s −1 component is detected at the westmost boundary of the remnant, which provides direct evidence of the SNR-MC interaction and suggests multi-component gas there with dense (∼ 10 4 cm −3 ) molecular clumps. We confirm the previous detection of a MC at ∼ 38 km s −1 to the west and south of the SNR and argue, based on HI self-absorption, that the cloud is located in the foreground of the remnant.A list of 64 Galactic SNRs presently known and suggested to be in physical contact with environmental MCs is appended in this paper.

Combining HI data from the Canadian Galactic Plane Survey (CGPS) and CO data from the Milky Way Imaging Scroll Painting (MWISP) project, we have identified a new segment of a spiral arm between Galactocentric radii of 15 and 19 kpc that apparently lies beyond the Outer Arm in the second Galactic quadrant. Over most of its length, the arm is 400-600 pc thick in z. The new arm appears to be the extension of the distant arm recently discovered by Dame & Thaddeus (2011) as well as the Scutum−Centaurus Arm into the outer second quadrant. Our current survey identified a total of 72 molecular clouds with masses on the order of 10 2 -10 4 M ⊙ that probably lie in the new arm. When all of the available data from the CO molecular clouds are fit, the best−fitting spiral model gives a pitch angle of 9.3 • ±0.7 • .

The Milky Way Imaging Scroll Painting (MWISP) project is an unbiased Galactic plane CO survey for mapping regions of l = −10 • to +250 • and |b| < ∼ 5. • 2 with the 13.7 m telescope of the Purple Mountain Observatory. The legacy survey aims to observe the 12 CO, 13 CO, and C 18 O (J=1-0) lines simultaneously with full-sampling using the nine-beam Superconducting SpectroScopic Array Receiver (SSAR) system with an instantaneous bandwidth of 1 GHz. In this paper, the completed 250 deg 2 data from l = +25. • 8 to +49. • 7 are presented with a grid spacing of 30 ′′ and a typical rms noise level of ∼ 0.5 K for 12 CO at the channel width of 0.16 km s −1 and ∼ 0.3 K for 13 CO and C 18 O at 0.17 km s −1 . The high-quality data with moderate resolution (∼50 ′′ ), uniform sensitivity, and high spatial dynamic range, allow us to investigate the details of molecular clouds (MCs) traced by the three CO isotope lines. Three interesting examples are briefly investigated, including distant Galactic spiral arms traced by CO emission with V LSR <0 km s −1 , the bubble-like dense gas structure near the H ii region W40, and the MCs distribution perpendicular to the Galactic plane.

OJ 287 is a BL Lac object at redshift z= 0.306 that has shown double‐peaked bursts at regular intervals of ∼12 yr during the last ∼40 yr. We analyse optical photopolarimetric monitoring data from 2005 to 2009, during which the latest double‐peaked outburst occurred. The aim of this study is twofold: firstly, we aim to analyse variability patterns and statistical properties of the optical polarization light curve. We find a strong preferred position angle in optical polarization. The preferred position angle can be explained by separating the jet emission into two components: an optical polarization core and chaotic jet emission. The optical polarization core is stable on time‐scales of years and can be explained as emission from an underlying quiescent jet component. The chaotic jet emission sometimes exhibits a circular movement in the Stokes plane. We find six such events, all on the time‐scales of 10–20 d. We interpret these events as a shock front moving forwards and backwards in the jet, swiping through a helical magnetic field. Secondly, we use our data to assess different binary black hole models proposed to explain the regularly appearing double‐peaked bursts in OJ 287. We compose a list of requirements a model has to fulfil to explain the mysterious behaviour observed in OJ 287. The list includes not only characteristics of the light curve but also other properties of OJ 287, such as the black hole mass and restrictions on accretion flow properties. We rate all existing models using this list and conclude that none of the models is able to explain all observations. We discuss possible new explanations and propose a new approach to understanding OJ 287. We suggest that both the double‐peaked bursts and the evolution of the optical polarization position angle could be explained as a sign of resonant accretion of magnetic field lines, a ‘magnetic breathing’ of the disc.

The explosive growth of massive data generation from Internet of Things in industrial, agricultural and scientific communities has led to a rapid increase for data analytics in cloud data centers. The ubiquitous and pervasive demand for near-data processing urges the edge computing paradigm in recent years. Edge computing is promising for less network backbone bandwidth usage and thus less data center side processing pressure, as well as enhanced service responsiveness and data privacy protection. Computation offloading plays a crucial role in edge computing in terms of network packets transmission and system responsiveness through dynamic task partitioning between cloud data centers and edge servers and edge devices. In this paper a thorough literature review is conducted to reveal the state-of-the-art of computation offloading in edge computing. Various aspects of computation offloading, including energy consumption minimization, Quality of Services guarantee, and Quality of Experiences enhancement are surveyed. Moreover, resource scheduling approaches, gaming and tradeoffing among system performance and overheads for computation offloading decision making are also reviewed.INDEX TERMS Edge computing, computation offloading, task partitioning, game theory, edge-cloud collaboration.

W49B is a mixed-morphology supernova remnant with thermal X-ray emission dominated by the ejecta. In this remnant, the presence of overionized plasma has been directly established, with information about its spatial structure. However, the physical origin of the overionized plasma in W49B has not yet been understood. We investigate this intriguing issue through a 2D hydrodynamic model that takes into account, for the first time, the mixing of ejecta with the inhomogeneous circumstellar and interstellar medium, the thermal conduction, the radiative losses from optically thin plasma and the deviations from equilibrium of ionization induced by plasma dynamics. The model was set up on the basis of the observational results. We found that the thermal conduction plays an important role in the evolution of W49B, inducing the evaporation of the circumstellar ring-like cloud (whose presence has been deduced from previous observations) that mingles with the surrounding hot medium, cooling down the shocked plasma, and pushes the ejecta backwards to the centre of the remnant, forming there a jet-like structure. During the evolution, a large region of overionized plasma forms within the remnant. The overionized plasma originates from the rapid cooling of the hot plasma originally heated by the shock reflected from the dense ring-like cloud. In particular, we found two different ways for the rapid cooling of plasma to appear: (i) the mixing of relatively cold and dense material evaporated from the ring with the hot shocked plasma and (ii) the rapid adiabatic expansion of the ejecta. The spatial distribution of the radiative recombination continuum predicted by the numerical model is in good agreement with that observed

It has been proposed that the charge exchange (CX) process at the interface between hot and cool interstellar gases could contribute significantly to the observed soft X-ray emission in star forming galaxies. We analyze the XMM-Newton/RGS spectrum of M82, using a newly developed CX model combined with a single-temperature thermal plasma to characterize the volume-filling hot gas. The CX process is largely responsible for not only the strongly enhanced forbidden lines of the Kα triplets of various He-like ions, but also good fractions of the Lyα transitions of C vi (∼ 87%), O viii and N vii ( 50%) as well. In total about a quarter of the X-ray flux in the RGS 6-30Å band originates in the CX. We infer an ion incident rate of 3 × 10 51 s −1 undergoing CX at the hot and cool gas interface, and an effective area of the interface as ∼ 2 × 10 45 cm 2 that is one order of magnitude larger than the cross section of the global biconic outflow. With the CX contribution accounted for, the best fit temperature of the hot gas is 0.6 keV, and the metal abundances are approximately solar. We further show that the same CX/thermal plasma model also gives an excellent description of the EPIC-pn spectrum of the outflow Cap, projected at 11.6 kpc away from the galactic disk of M82. This analysis demonstrates that the CX is potentially an important contributor to the X-ray emission from starburst galaxies and also an invaluable tool to probe the interface astrophysics.