  The International X-ray Observatory (IXO) is a joint NASA-ESA-JAXA effort.
X-ray observations will resolve pressing astrophysical questions such as: What
happens close to a black hole? How do supermassive black holes grow? How does
large scale structure form? What is the connection between these processes? To
address these questions requires dramatic increases in collection area combined
with sensitive new instrumentation. IXO's spectroscopic, timing, and
polarimetric capabilities will probe close to the event horizon of
super-massive black holes (SMBH) where strong gravity dominates. IXO will
determine the evolution and origin of SMBH by measuring their spin to
understand their merger history, surveying them to find their luminosity
distribution out to high redshift (z~8), and spectroscopically characterizing
their outflows during peak activity. IXO will revolutionize our understanding
of galaxy clusters by mapping their bulk motions and turbulence. IXO will
observe the process of cosmic feedback where black holes inject energy on
galactic and intergalactic scales, and characterize the missing baryons in the
cosmic web. Meanwhile, surveys of distant clusters will constrain cosmological
models. IXO will be available to all astronomers, taking X-ray astrophysics
from an era where high-resolution spectra are a rarity to one with vast numbers
of spectra from all types of sources. Powerful spectral diagnostics and large
collecting areas will reveal unexpected discoveries, with IXO studying new
phenomena as they appear-a key feature of great observatories.
