  The International X-Ray Observatory (IXO) will address fundamental questions
in astrophysics, including "When did the first SMBH form? How does large scale
structure evolve? What happens close to a black hole? What is the connection
between these processes? What is the equation of state of matter at
supra-nuclear density?" This report presents an overview of the assessment
study phase of the IXO candidate ESA L-class Cosmic Vision mission. We provide
a description of the IXO science objectives, the mission implementation and the
payload. The performance will offer more than an order of magnitude improvement
in capability compared with Chandra and XMM-Newton. This observatory-class
facility comprises a telescope with highly nested grazing incidence optics with
a performance requirement of 2.5 sq.m. of effective area at 1.25 keV with a 5"
PSF. There is an instrument complement that provides capabilities in imaging,
spatially resolved spectroscopy, timing, polarimetry and high resolution
dispersive spectroscopy. Since earlier submissions to the Astro2010 Decadal
Survey, substantial technological progress has been made, particularly in the
mirror development. Risk reduction measures and important programmatic choices
have also been identified. An independent internal Technical and Programmatic
Review has also been carried out by ESA, concluding with positive
recommendations. Subject to successful conclusion of agreements between the
partner space agencies, IXO is fully ready to proceed to further definition,
moving towards an eventual launch in 2021-2022.
