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........ published in NEWSLETTER # 67

by Dr. R.Buchheri, Consiglio Nazionale delle Ricerche, Palermo (Italy)

This NATO ASI was the fourth in a series focusing on the astrophysics of neutron stars, which started with the ASI "Timing neutron stars" (1988) and was followed by "Neutron stars: Theory and Observations" (1990) and "The lives of the neutron stars" (1993).

During the last several years the number of neutron stars in their various disguises (e.g. millisecond radio pulsars, single neutron stars, X-ray sources, soft gamma-ray repeaters, ...) has increased substantially and new phenomena have been discovered in known populations of neutron star systems (.e.g. QPO in low binaries, evaporation by millisecond pulsars, presence in globular clusters), thus determining the choice of "The many faces of neutron stars" as the theme of this ASI, held in Sicily, October 1996.

The existence of Neutron Stars, theoretically predicated since 1934 by Baade and Zwicky, was definitely accepted in the late sixties, after the exciting discovery of pulsars by Antony Hewish and Jocelyn Bell at Cambridge University.

It is now well established that they form at the end of the evolutionary process to which relatively massive stars are subject following their gravitational collapse and subsequent thermonuclear radiation. It is also accepted that the great amount of spin gained at the end of the collapse together with the large superficial magnetic field, allow relativistic charged particles to be extracted from the surface of the so formed neutron stars and, by interaction with the magnetic field and/or with the surrounding matter, emit electromagnetic radiation, from radio to gamma rays, observable from the earth in special geometrical conditions of view.

The precise processes giving rise to such radiation are not, however, completely known. They depend very much on the specific case involved, for intrinsic reasons (if the object is isolated or in a binary system, if it is large or small, if it is at the beginning or at the end of the evolution), or for particular observational conditions (which photon energy are we looking at and for how long). Interdisciplinary studies have therefore developed aiming to understand the underlying physical mechanisms.

From the observational point of view, for example, recent studies following the detection of gamma-ray bursts have opened a new front connected with the behaviour of Neutron Stars at the early stage of their evolution. Also, studies of the description of the internal structure of Neutron Stars and the structure and evolution of the external magnetic field are continuously advancing.

It was timely to summarize at the ASI, and present in tutorial form to interested graduate students, all the information gathered about Neutron Stars in many different aspects (NATO ASI SERIES C515) ranging from theoretical models developed on their formation and evolution, on their supposed radiation mechanisms, on the behaviour of high density matter, on magnetic field evolution, etc. ...., to instrumental and analysis techniques, thus improving our understanding of the astronomical observations.
Reference books: C262, C300, C344, C450, C515

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