Acknowledgments

This thesis is the summary of a three years work in the field of astrophysics, and I'd like to thank all the people who has contributed to it.
I'm deeply indebted to my supervisor Sandro Mereghetti, for his attention in following my activities and for his constant friendship.
I wish to acknowledge Marco Tavani for the useful discussions and, above all, for his continuous encouragement.
I'm really grateful to all the Milano AGILE Group for the friendship they have always demonstrated.
I heartily thank Stan Hunter for his great availability in sharing his experience during useful discussions which have helped me in improving my model. I'm most grateful to Andrew Strong for his constructive suggestions about this thesis and for his great kindness.