Preface

When the existence of neutron stars was confirmed by the discovery of radio pulsars in August 1967, there was general optimism that it should not be too difficult to explore and understand the physical properties of a rotating magnetised compact star with ∼10 km radius. Forty years and more than 13 PhD student-generations later, everybody involved in the neutron star business has lost this illusion, meanwhile learning how complex neutron stars are and how difficult it is to understand their physical properties. Neutron stars form in supernova explosions and/or by an accretion induced collapse of a white dwarf. At the time of their discovery – and for many years later – it was generally accepted that neutron stars can only be observed as pulsars. According to the source of energy they were split into two classes, i.e. being powered by either rotation or accretion. Today, the neutron star world is much more intrincate than it was fourty decades ago. In addition to the accretion powered pulsars, which are predominantly bright X-ray sources, and the rotation-powered pulsars which are observed throughout the electromagnetic spectrum, there are now X-ray Dim Isolated Neutron Stars (XDINs), “radio-quiet neutron stars”, Compact Central Objects (CCOs) in supernova remnants, Soft Gamma-ray Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs). Accordingly, neutron stars manifest themselves in many different ways. They become visible by high-energy processes occurring on their surface or surrounding region. In most of these objects, ultra-strong magnetic fields are a crucial element in the radio, optical, X-ray and gamma-ray emission processes which dominate the observed spectrum. Observationally, neutron star research is advancing steadily. A great array of space instruments (the Hubble Space Telescope, ROSAT, ASCA, BeppoSAX, RXTE and the Compton Gamma-Ray Observatory), launched in the last decade of the twentieth century have opened new windows on neutron star research with high quality data in energy bands from the optical to gamma-rays. With the more recently launched satellite X-ray observatories Chandra and XMM-Newton, the H.E.S.S. Array of Imaging Atmospheric Cherenkov Telescopes, upgraded radio observatories and ground based optical telescopes a number of questions which