During the last three years I was scattered in three different towns in Italy (good, but unluckily not connected by fast trains) and in each of them I met many peoples, the great majority of whom I can now call friends, that helped and supported me in many ways.
Trieste: I am deeply in debt to Aldo Treves for his constant supervision of my work, continuous encouragement during the time in which this research project has been developed as well as for financial support on many occasions. I wish also to thank Dennis Sciama, head of the Astrophysics Sector of SISSA, always ready to help me in any way, whenever I needed it. Antonio Lanza generously offered me his scientific experience and friendship. I would like to thank also the scientific and administrative staff of the SISSA.
Milan: My deep thanks go to Luigi Stella for teaching me most of the things I know about X-ray data analysis, for his constant support and patience, and for putting me on the way of good research. Thanks also for allowing me to test lemon Simmenthal and Muccarella. Sandro Mereghetti and Giampiero Tagliaferri and Monica Colpi have always generously offered me their scientific experience and friendship. I also thank the scientific and administrative staff of the Milan Astronomical Observatory (both in Brera and Merate).
Rome: At the ``La Sapienza'' Rome University many have helped me. In particular the G9 group and Prof. Remo Ruffini supported me in many ways in the last few years. I wish also to thank Franca D'antona, Amedeo Tornambè and Mario Vietri for many useful discussions, and the scientific and administrative staff of the Rome Astronomical Observatory (Monteporzio Catone).
In addition, many other people in many parts of the world have helped me. I would like to thank Lorella Angelini (NASA-GSFC), Nick White (NAS-GSFC) and Paolo Giommi (SAX SDC) for providing me (at very short notice) the FITS light curves of the WGA catalogue and for their scientific collaboration. Many thanks go also to Tim Kallman (NASA-GSFC) for his kind scientific availability. Thanks to the HEASARC staff who helped me to solve many problems met during ROSAT data analysis over the last 3 years.
I am also grateful to Michael van der Klis (UvA) for fruitful discussions about the coherent signal detection technique used in this work.
I wish also to thank Arvind Parmar (ESTEC-SSD) for his help in the EXOSAT ME data analysis and for many useful discussions. Many thanks go to Alastair Reynolds (ESTEC-SSD) who helped me in solving many problems met during EXOSAT data timing analysis.
I would like to thank Tomaso Belloni (UvA) for initiating me into the ROSAT secrets, for his scientific collaboration and for deep and fruitful discussions about the books of Pennac and Benni.
I'm indebted to Prof. J. Grindlay (CfA) and J. C. Wheeler (University of Texas) for allowing me to participate to the 96 Aspen Winter Conference, despite the short notice, and to report part of the results included in this thesis. I'm also grateful to Prof. F. Primini (CfA) who allowed me to analyse his ROSAT HRI data before publication of the results.
I am grateful to Stefano Covino, Renato Falomo, Alessandro Pizzella and Luca Reduzzi for optical follows-up (ESO-La Silla, Chile) of some of the X-ray sources included in this work.
In addition, I would like to thank all the friends whom I met in Trieste, Milan and Rome over these three years. In strictly stochastic order: Enzo, Leo, Silvia, Beppe, Giovanni, Elena (three more or less), Frasio, Fabio, Max, Alejandra, Janmin, Marco, Valentina, Stefano (actually they are five), Alice, Marly, Lorenzo, Carlo, Mercedes, Gloria, Fabia, Radikkio, Bruce, Francesco, Cristina, Salvatore, Donato, Marcello, Daniele, Colleen & Chloe, Davide, Alberto, Pim & Gianantonio, Rachele, Marco & Giulia, Paolo, Rosy, Agostino, and many others.
Special thanks go to Donatella, Giovanni, Piero, Sergio, Silvia and Stefano (QN): they enriched my life both at and away from work.
An early version of this thesis was carefully read by many friends which drastically reduced the number of misspellings. Blame me for the still unfound ones.
I am deeply grateful to Elisabetta for her constant presence and help. The final thought is for my family who have always believed in me and supported me in many ways.
All these people and many others contributed to make unforgettable these three years. Thanks very much !!
``This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA-Goddard Space Flight Center.''
``This research has made use of data obtained through the EXOSAT on-line Interactive Analysis provided by ESA-ESTEC, Space Science Division''.