Observational constraints on Be/X-ray binary models

This work deals with the study of Be/X-ray binaries using a multi-wavelength approach and the information we can extract from the observations. It is shown that long-term multi-wavelength monitoring allows us to put constraints on the different models that have been proposed to account for the properties of these systems.Detailed spectroscopic and photometric analysis is used to establish the accurate spectral classification of the optical components of the Be/X-ray binaries 3A 0726-26, 4U 2206+54 and 4U0115+634. The three objects are shown to be late O-type stars close to the main sequence and presenting emission contamination in most lines in the optical range. The case of 4U2206+54 is somewhat special, since it does not seem to correspond exactly with any spectral type, arousing suspicions of possible binarity of the optical star.The multi-wavelength techniques are then applied to the recurrent transient pulsator 4U0115+634. Hard X-ray data are presented and their correlation to optical/infrared observations studied. The complex optical/infrared/X-ray behaviour is explained in terms of a series of mass ejection events that lead to the observed outbursts. The degree of variability of the source is shown to be much higher than that of similar objects, which is attributed to the close passage of the neutron star through the circumstellar environment.Spectroscopy in the optical range reveals that the discs surrounding the optical components of the Be/X-ray binaries A 0535+26 and 4U 0115+634 were strongly disturbed in coincidence with major X-ray outbursts. It is shown that observational evidence strongly points to the frequent presence of large-scale perturbations in the circumstellar envelopes of Be/X-ray binaries. Different ways in which these density asymmetries can arise are discussed. The role of the interaction between the neutron star and the circumstellar material is shown to be probably very important. A consistent model explaining the different types of X-ray activity in Be/X-ray binaries is presented.