H. Bradt

R. Giacconi

The IAU Symposium No. 55 on 'X-Ray and Gamma-Ray Astronomy' has occurred, not entirely by coincidence, at an important moment in the development of these new branches of observational astronomy. In X-ray astronomy the data from the first X-ray observatory UHURU have contributed to a new view of the X-ray sky and a new conception of the nature and properties of galactic and extragalactic X-ray sources. In gamma-ray astronomy the exciting and often controversial nature of the results underlines the importance of the forthcoming launch of SAS-B, the first orbiting y-ray observatory. As Bruno Rossi reminds us (p. I), the Symposium occurred almost exactly ten years after the first detection of the X-ray star Sco X-I. During this time we have moved from the detection of a handful of the nearest and brightest sources to the detailed study of the nature of stellar sources in the farthest reaches of our own galaxy and in external galaxies of the local group. The detection of pulsating X-ray sources in bi nary systems permits the measurement of pulsation periods, and orbital parameters with precisions comparable to any yet achieved with traditional observational techniques. The strong indications that most X-ray sources are extremely compact objects give us confidence that X-ray astronomy will playa significant and possibly decisive role in the study of stars near the end point of stellar evolution.

X- and Gamma-Ray Astronomy

L. Scarsi

P. Giommi

F. Fiore

The conference on Active X-Ray Sky was held approximately two years after the launch of the Rossi-Xte and RXTE satellites. The objective of these proceedings was to incorporate results not only from these two missions, but to review recent developments about the Active X-ray Sky as obtained from the four currectly active X-ray missions, as well as from the Copton Gamma-ray Observatory. Some of the very dramatic findings were: the discovery of X-ray and optical afterglow in Gamma Ray Bursts; the discovery of X-ray oscillations at kilohertz frequencies from binary systems and the discovery of correlated IR flares, radio flares and X-ray dip-flare cycles in microquasars. These results have yielded dramatic new insights into our physical universe. Other important results reported or reviewed here include the detection of extreme high-energy emission in the active binary stars Algol and UX Ari, resonant absorption scattering from the Perseus Cluster, and cyclotron lines from pulsars. These and many other recent and important results are published here, resulting in a state-of-art review by experts currently active in the field.

H. Bradt

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