The observability of isolated neutron stars and black holes.

The detection of old neuron stars and black holes in isolation is one of the cornerstones of compact object astrophysics. However, forty years after the first pioneering studies, no succesful candidates have been found to confirm the early predictions, making the search for old isolated compact objects a still open and intriguing subject. The scope of this thesis is thus to investigate the observability of isolated neutron stars and black holes with the final objective of defining new possible strategies for the long sought detection of these elusive objects. The thesis is organized as follows. In Chapter 1, after a short discussion on the origin of neuron stars and stellar mass black holes in isolation, I summarize the results of past efforts made to constrain their observational properties. In Chapter 2 I tackle with the dynamics of isolated neutron stars. I describe the set up of a numerical Monte Carlo code, named Population Synthesis of Compact Objects (PSYCO in brief), developed as part of the PhD project. I then present the results of the simulation with particular emphasis for statistical properties of neutron stars in the Galactic disk and in the solar neighbourhood. These first results will be used as base to explore alternative methods to detect old neutron stars and black holes. It should be noted that, following the standard practice, these first results are obtained considering only neutron stars born in the disk of the Milky Way. As it will be shown in Chapter 3, the contribution of neutron stars, and black holes, born in the Galactic bulge cannot be neglected since they could represent the majority of detectable objects. In Chapter 3 the feasibility of microlensing as a technique to detect isolated neutron stars and black holes is explored, making use of the results illustrated in the previous Chapter. After an overview of results obtained so far by the several surveys, I describe the basic microlensing quantities and expressions. I then describe the models adopted in my work for the distribution of bulge and disk stars, to which the contribution of neutron stars and black oles is then compared to. I compare the optical depth and event rate due to neutron stars and black holes with that of normal stars. Also, I study the distribution of event time scales in both cases. After the results reported in Chapter 3, in Chapter 4 I present a systematic cross correlation analysis of microlensing events with the catalogues of X-rays sources of the XMM-Newton and Chandra satellites, which appeared recently. I report the results of the cross correlation and the properties of a source resulting from the cross correlation procedure. Finally, in Chapter 5 I review the results of my PhD project and draw the final conclusion. I also discuss possible future developments.