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Athena (Advanced Telescope for High ENergy Astrophysics) Assessment Study Report for ESA Cosmic Vision 2015-2025

Athena is an X-ray observatory-class mission concept, developed from April to December 2011 as a result of the reformulation exercise for L-class mission proposals in the framework of ESA's Cosmic Vision 2015-2025. Athena's science case is that of the Universe of extremes, from Black Holes to Large-scale structure. The specific science goals are structured around three main pillars: "Black Holes and accretion physics", "Cosmic feedback" and "Large-scale structure of the Universe". Underpinning these pillars, the study of hot astrophysical plasmas offered by Athena broadens its scope to virtually all corners of Astronomy. The Athena concept consists of two co-aligned X-ray telescopes, with focal length 12 m, angular resolution of 10" or better, and totalling an effective area of 1 m2 at 1 keV (0.5 m2 at 6 keV). At the focus of one of the telescopes there is a Wide Field Imager (WFI) providing a field of view of 24'\times 24', 150 eV spectral resolution at 6 keV, and high count rate capability. At the focus of the other telescope there is the X-ray Microcalorimeter Spectrometer (XMS), a cryogenic instrument offering a spectral resolution of 3 eV over a field of view of 2.3' \times 2.3'. Although Athena has not been selected as ESA's Cosmic Vision 2015-2025 L1 mission, its science goals and concept conform the basis of what should become ESA's X-ray astronomy flagship.