In the early years of X-ray astronomy, one of us (J. E. T.) had always a book in reach, entitled "X-ray Astronomy" and edited by Riccardo Giacconi and Herbert Gursky about a decade after they had opened the field by the discovery of Scorpius X-1 and the X-ray background. This book summarized all the knowledge at the time, based on the results from the pioneering rocket and balloon experiments and from Uhuru, the first satellite entirely dedicated to X-ray astronomy.

Since those early times X-ray astronomy has evolved with enormous pace. The number of known sources has increased by a factor of thousand, but more important, they now comprise almost all classes of astronomical objects - from planets, moons and comets out to clusters of galaxies and quasars. In the era of multi-wavelength astronomy X-ray observations provide insight into extreme physical conditions prevailing in all these sources - very high temperatures, very strong gravitational fields, super-nuclear densities, extreme concentrations of relativistic particles.

The intent of this book is to summarize the present status of the field, which has become quite challenging, since the number of publications in refereed journals has risen to more than 20 000. Therefore the coverage cannot be complete, but must rather be representative. We apologize for omitting any important ideas, methods or results.

The authors of the various chapters are mainly scientists working at the MaxPlanck-Institute for Extraterrestrial Physics, the home of ROSAT, or colleagues who have been working closely with us during the last 20 years. Besides ROSAT, the main sources of information have been the other X-ray satellites of the nineties - ASCA, RXTE and BeppoSAX - and their more recent successors - Chandra, XMM-Newton, INTEGRAL, Swift and Suzaku - which have used novel instrumentation to produce a wealth of knowledge on the universe seen at high energies.

This book addresses mainly scientists who are teaching the subject, and young scientists entering the field, as well as astronomers from neighbouring disciplines and physicists interested in one of the most exciting fields of astrophysics. It is organized in a straightforward way: We start with a discussion of instruments and methods in part I and then continue in parts II and III with the status of galactic and extragalactic X-ray astronomy respectively, ordering the contributions in a geocentric fashion. In Chapter 26 a short summary of the current plans for future missions in X-ray astronomy is given.

We are very thankful to all authors of this book for their contributions. The editorial assistance of Konrad Dennerl who brought the LaTeX manuscript into its final shape is gratefully acknowledged. We thank Birgit Boller and Walburga Franken-huizen for their dedicated secretarial support, as well as Maria Fiirmetz and Barbara Mory for their painstaking work on the index of the book.

Garching, November 2007

Joachim E. Triimper Gunther Hasinger

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