aReseareli and Scientific Support Department of ESA, ESTEC, Noordwijk, The Netherlands bAstrophysikalisches Institut Potsdam, 14482 Potsdam, Germany CCEA/DSM/DAPNIA/SAP CEN-Saclay, Gif-sur-Yvette, France dlnstituto de Fisica de Cantabria (CSIC-UC), Santander, Spain eCE5RCNR5/Up5, Toulouse, France Observatoire Astronomique de Strasbourg, Strasbourg, France MP für Extraterrestrische Physik, Garching bei Mflnchen, Germany hlstituto TESRE, CNR, Bologna, Italy Osservatorio Astronomico di Bologna, 40127 Bologna, Italy 'Observatoire de Geneve, Sauverny, Switzerland klnstitute of Astronomy, Madingley Road, Cambridge, UK 11A&A, National Obs. of Athens, Greece tmCarnegie Mellon University, Pittsburgh, PA, USA RIKEN, Wako, Saitama, Japan °Kyoto University, Sakyo, Kyoto, Japan PDept of Physics, University of Tokyo, Bunkyo-ku, Tokyo, Japan Mullard Space Science Laboratory, UCL, Dorking, UK rQbservatojre de Strasbourg, France 8SRON, Utrecht, The Netherlands tTokyo Metropolitan University, Hachioji, Tokyo, Japan UColumbia Univesity, New York, USA vlstituto di Astrofisica Spaziale (lAS), CNR, 00133 Rome, Italy WUnjversjtãt Hamburg, Hamburger Sternwarte, Hamburg, Germany xAstronomical Institute Anton Pannekoek, University of Amsterdam, The Netherlands YXray Astronomy Group, University of Leicester, Leicester, UK