ROSAT Observations G. HASINGER Max-Planck-Institut flir extraterrestrische Physik, D-85740 Garching, Germany Abstract. This review describes the most recent advances in the study of the extragalactic soft X-ray background and what we can learn about its constituents. The deepest pointed observations with the ROSAT PSPC are discussed. The logN-logS relation is presented, which reaches to the faintest X-ray fluxes and to the highest AGN surface densities ever achieved. The N(>S) relation shows a 2 density in excess of 400 deg- at the faintest fluxes and a flattening below the Einstein Deep Survey limit. About 60% of the extragalactic background has been resolved in the deepest field. Detailed source spectra and first optical and radio identifications will be discussed. The results are put into perspective of the higher energy X -ray background. Key words: X-rays, background radiations, active galactic nuclei. 1. Introduction The extragalactic X-ray background (XRB), discovered about 30 years ago, has been studied extensively with many X-ray experiments, in particular with the satel lites HEAO I and II (see ego Boldt 1987) and with ROSAT (e. g. Hasinger et aI. , 1993). Figure 1 shows a compilation of some of the most recent spectral measure ments for the X-ray background. Over the energy range from 3 to about 100 keY its spectrum can be well approximated by an optically thin thermal bremsstrahlung model with kT ~ 40 keY, while at lower X-ray energies a steepening into a new component has been observed observed (e. g.

In this original and integrated approach to theoretical reasoning in physics, Malcolm Longair illuminates the subject from the perspective of real physics as practised by research scientists. Concentrating on the basic insights, attitudes and techniques that are the tools of the modern physicist, this approach conveys the intellectual excitement and beauty of the subject. Through a series of seven case studies, an undergraduate course in classical physics and the discovery of quanta are reviewed from the point of the view of how the great discoveries and changes of perspective came about. This approach illuminates the intellectual struggles needed to attain understanding of some of the most difficult concepts in physics. Longair's highly acclaimed text has been fully revised and includes new studies on the physics of fluids, Maxwell's great paper on equations for the electromagnetic field and problems of contemporary cosmology and the very early universe.

Delineating the huge strides taken in cosmology in the past ten years, this much-anticipated second edition of Malcolm Longair's highly appreciated textbook has been extensively and thoroughly updated. It tells the story of modern astrophysical cosmology from the perspective of one of its most important and fundamental problems - how did the galaxies come about? Longair uses this approach to introduce the whole of what may be called "classical cosmology". What's more, he describes how the study of the origin of galaxies and larger-scale structures in the Universe has provided us with direct information about the physics of the very early Universe.

The twentieth-century witnessed the development of astrophysics and cosmology from subjects which scarcely existed to two of the most exciting and demanding areas of contemporary scientific inquiry. In this book Malcolm Longair reviews the historical development of the key areas of modern astrophysics, linking the strands together to show how they have led to the extraordinarily rich panorama of modern astrophysics and cosmology. While many of the great discoveries were derived from pioneering observations, the emphasis is upon the development of theoretical concepts and how they came to be accepted. These advances have led astrophysicists and cosmologists to ask some of the deepest questions about the nature of our Universe and have pushed astronomical observations to the very limit. This is a fantastic story, and one which would have defied the imaginations of even the greatest story-tellers.

ROSAT Observations G. HASINGER Max-Planck-Institut flir extraterrestrische Physik, D-85740 Garching, Germany Abstract. This review describes the most recent advances in the study of the extragalactic soft X-ray background and what we can learn about its constituents. The deepest pointed observations with the ROSAT PSPC are discussed. The logN-logS relation is presented, which reaches to the faintest X-ray fluxes and to the highest AGN surface densities ever achieved. The N(>S) relation shows a 2 density in excess of 400 deg- at the faintest fluxes and a flattening below the Einstein Deep Survey limit. About 60% of the extragalactic background has been resolved in the deepest field. Detailed source spectra and first optical and radio identifications will be discussed. The results are put into perspective of the higher energy X -ray background. Key words: X-rays, background radiations, active galactic nuclei. 1. Introduction The extragalactic X-ray background (XRB), discovered about 30 years ago, has been studied extensively with many X-ray experiments, in particular with the satel lites HEAO I and II (see ego Boldt 1987) and with ROSAT (e. g. Hasinger et aI. , 1993). Figure 1 shows a compilation of some of the most recent spectral measure ments for the X-ray background. Over the energy range from 3 to about 100 keY its spectrum can be well approximated by an optically thin thermal bremsstrahlung model with kT ~ 40 keY, while at lower X-ray energies a steepening into a new component has been observed observed (e. g.

Providing students with an in-depth account of the astrophysics of high energy phenomena in the Universe, the third edition of this well-established textbook is ideal for advanced undergraduate and beginning graduate courses in high energy astrophysics. Building on the concepts and techniques taught in standard undergraduate courses, this textbook provides the astronomical and astrophysical background for students to explore more advanced topics. Special emphasis is given to the underlying physical principles of high energy astrophysics, helping students understand the essential physics. The third edition has been completely rewritten, consolidating the previous editions into one volume. It covers the most recent discoveries in areas such as gamma-ray bursts, ultra-high energy cosmic rays and ultra-high energy gamma rays. The topics have been rearranged and streamlined to make them more applicable to a wide range of different astrophysical problems.

The significance of the present IAU symposium, "The Large Scale Structure of the Universe," fortunately requires no elaboration by the editors. The quality of the wide range of observational and theoretical astrophysics contained in this volume speaks for itself. The published version of the proceedings contains all the contributions presented at the symposium with the exception of the introductory lecture by V. A. Ambartsumian. Contributed papers, short contributions and discussions have been included according to the recommendations of the IAU. Many people contributed to the success of the symposium. First of all, thanks are due to the USSR Academy of Sciences and to the Estonian Academy of Sciences for sponsoring this symposium in Tallinn. The efforts of Academician K. Rebane, President of the Estonian Academy of Sciences, are particularly appreciated. The astronomical hosts of the symposium were the members of the W. Struve Astrophysical Observatory of Tartu who made outstanding efforts to lavish participants with Estonian hospitality which was greatly appreciated and enjoyed by them and their guests. The members of the Scientific and Local Organising Committees are listed below and we thank all of them for their contributions which were central to the success of the symposium. In addition are listed members of the Technical Organising Committee who were responsible for all details of the organisation and whose vigilance ensured that all aspects of the symposium ran smoothly and efficiently. Their contributions are all gratefully acknowledged.

be hoped. We have improved measurements by at best a factor of 3. So thIS paper, unfortunately, IS really a status report rather than a progress report. There are, however, a few new results to mention. I shall present the results wIthout glVlng any detaIls of the experImental apparatus. In general, sources of error and problems in these measurements are not determined by the apparatus itself. The most important new result IS that of Paul Henry (1971), who employed a radiometer mounted on a rotatmg platform suspended beneath a balloon. Because he employed a rotatmg platform he was able to look for amsotropy over a WIde area area of the sky (about one-half of the northern hemIsphere), not Just a CIrcle of con- stant dechnatIOn. In a smgle mght, he was able to obtam enough data to establIsh a value for the component of the 'dIpole' anisotropy parallel to the spm aXIS of the Earth It IS LI T = (3. 2 +- 0 8) x 10-3 K in the direction (X = I Oh-ll hand () = - 30 . HIS results are conSIstent WIth the earher results of Conkhn (1969), but provIde the Im- portant addItIonal datum that the motion of the Earth with respect to the co-movmg coordinate system (and parallel to the spm aXIS of the Earth) IS small. MeanwhIle, Conklin refined and repeated hIS earher measurement and reduced the statIstical error. The results of his work are reported m the IAU Symp. 44 (1972).

The twentieth-century witnessed the development of astrophysics and cosmology from subjects which scarcely existed to two of the most exciting and demanding areas of contemporary scientific inquiry. In this book Malcolm Longair reviews the historical development of the key areas of modern astrophysics, linking the strands together to show how they have led to the extraordinarily rich panorama of modern astrophysics and cosmology. While many of the great discoveries were derived from pioneering observations, the emphasis is upon the development of theoretical concepts and how they came to be accepted. These advances have led astrophysicists and cosmologists to ask some of the deepest questions about the nature of our Universe and have pushed astronomical observations to the very limit. This is a fantastic story, and one which would have defied the imaginations of even the greatest story-tellers.

"Das erklarte Universum" ist eine anschauliche, laiengerechte und begeisternde Einfuhrung in die heutigen Kenntnisse der Astronomie und Kosmologie. Mit neuesten Farbbildern des leistungsfahigsten Teleskops der Welt bietet das Buch einen farbenprachtigen Blick auf unser Universum. Malcolm Longair nimmt uns mit auf eine atemberaubende Reise hin zu Entdeckungen, die Forscher uber die Geburt der Sterne, auf der Jagd nach schwarzen Lochern und dunkler Masse, bei Beobachtungen mit Gravitationslinsen und neuesten Tests im Zusammenhang mit dem Urknall gemacht haben. Der Leser lernt die wesentlichen Fragen, die sich die zukunftige Weltraumforschung stellen und die sie beantworten muss, verstehen.

"Dies ist kein Lehrbuch der theoretischen Physik, auch kein Kompendium der Physikgeschichte ..., vielmehr eine recht anspruchsvolle Sammlung historischer Miniaturen zur Vergangenheit der theoretischen Physik - ihrer "Sternstunden," wenn man so will. Frei vom Zwang, etwas Erschopfendes vorlegen zu mussen, gelingt dem Autor etwas Seltenes: einen "lebendigen" Zugang zum Ideengebaude der modernen Physik freizulegen, ... zu zeigen, wie Physik in praxi entsteht... Als Vehikel seiner Absichten dienen dem Autor geschichtliche Fallstudien, insgesamt sieben an der Zahl. Aus ihnen extrahiert er das seiner Meinung nach Lehrhafte, dabei bestrebt, mathematische Anachronismen womoglich zu vermeiden... Als Student hatte ich mir diese gescheiten Essays zum Werden unserer heutigen physikalischen Weltsicht gewunscht. Sie sind originell, didaktisch klug und genieren sich auch nicht, von der Faszination zu sprechen, die ... von der Physik ausgeht. Unnotig darauf hinzuweisen, das sie ein grundliches "konventionelles" Studium weder ersetzen wollen noch konnen, sie vermogen aber, dazu zu ermuntern." #"Astronomische Nachrichten (zur englischen " "Ausgabe)"#1"