We are currently in a"golden age" of astrophysics. Several fundamental discoveries have led to paradigm shifts in recent years. Among the most important are the establishment of a high precision cosmological "Standard Model," with dark energy, dark matter, and hot baryons as the dominating forms of the matter/energy density, and the confirmation of an inflationary phase in the early Universe, the discovery of extrasolar planets, the understanding that Gamma ray bursts mark the birth of black holes in distant galaxies, and the realization that supermassive black holes are an essential element of galaxy formation and evolution. Astrophysics has become an integral component of modern physics and leads the way to concepts beyond the standard models of elementary particle physics and relativity. X-ray astronomy is a crucial element of modern astrophysics and is particularly well suited to study matter under extreme conditions, black holes, and warm/hot baryons in the dark matter potential wells of the cosmic web.

The current observational capabilities for X-ray astronomy are excellent. The two X-ray observatory work horses, NASA's Chandra and ESA's XMM-Newton, are still performing extremely well after more than 7 years in orbit and we just can support every effort and hope that they will continue operation for the next several years. NASA's RXTE and ESA's INTEGRAL satellites have complementary capabilities in the hard X-ray energy range. The NASA satellite Swift has been launched in November 2004 with important European instrument contributions and is now routinely chasing gamma ray bursts and other violent phenomena "on the fly." The Japanese satellite Suzaku with important US instrument contributions been launched in July 2005 and offers an unprecedented sensitivity over the widest X-ray band pass. With four working satellites equipped with imaging X-ray telescopes and several nonimaging detectors in orbit, research possibilities in the fields covered by this book are very bright in the next few years. The goal of high-resolution nondispersive X-ray spectroscopy is still a very high priority for the future.

Both NASA and ESA are discussing next generation X-ray observatories Constellation-X and XEUS, respectively, which either separately or joined to a next generation global X-ray observatory, should surpass the sensitivity of the instruments on the current major observatories as well as the intermediate dedicated missions planned for the next decade by more than an order of magnitude.

This sensitivity is well matched to future observatories in other wavebands, e.g., the extremely large groundbased telescope(s) (ELT) in the optical, the James Webb space telescope (JWST) in the near-infrared, the Atacama large millimeter array (ALMA) in the submillimeter or the square kilometer array (SKA) in the radio band. The time interval between large space missions in any wave band and in any space agency are very long (15-25 yrs) and significant technology development is required from one step to the next. In order to maintain the scientific competitiveness and the technological expertise of scientific groups worldwide, as well as the excitement and career possibilities of young scientists, it is of utmost importance to develop intermediate, smaller projects like those planned in Japan, France, Italy, Germany, UK, Russia, USA, Brasil, India, and China. Already approved, under development, or close to launch are the two gamma-ray satellites AGILE (ASI) and GLAST (NASA), the Indian mission astrosat, the Brasilian mission MIRAX, the Japanese all-sky monitor MAXI and their next X-ray telescope mission (NeXT), the Chinese hard X-ray mission HXMT, the French/Italian/German formation flying mission SIMBOL-X, and the revived Russian Spektrum-Roentgen-Gamma mission SRG, with significant instrument contributions from Germany (eROSITA) and other partners. These missions, addressing exciting scientific topics like dark energy, dark matter, and the cosmic Web, the complete census of obscured black holes and time variable phenomena, will hopefully be launched before 2012.

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