One of the key issues in the investigation of Be/X-ray binaries is the study of the interaction between the neutron star and the circumstellar disk of the optical companion, The Figure below shows the maximum value of the H alpha equivalent width as a function of the orbital period for a number of Be/X-ray bineries. The H alpha equivalent width gives a measure of the size of the circumstellar envelope of the Be star. Roughly, the higher the equivalent width, the bigger the envelope. Thus, this diagram shows that systems with short orbital periods, that is, narrow orbits have smaller disks than those Be/X-ray systems with long orbital periods, i.e. wide orbits.
Fig caption: EW of H" as a function of orbital period. The arrows indicates lower limits for two systems. The orbital periods of the two systems represented by pink squares is unknown but has been deduced from the Corbet Ppulse- Porbit diagram for Be/X-ray systems. This figure is taken from Reig et al (1997 A&A 322, 193).
We attribute the existence of this relationship to the presence of the neutron star which prevents its optical counterpart from developing a large circumstellar disk. The neutron star is continuously orbiting round the Be star and when the disk radius approximately equals the binary separation, the compact star interacts with the disk and disrupts it. Since the separation between the two components of a binary system is proportional to the orbital period, the systems with the longer orbital periods can develop larger disks.
Asymmetric double-peak emission lines are usually described by their V/R ratio, defined as the ratio of violet-side to red-side peak intensities above continuum in units of continuum intensity. The figure below shows the long-term variability of the Halpha line in the Be/X-ray binary LS I+61 235. The period of the V/R variations is 3 years approximately:
These V/R variability can be explained in terms of the global one-armed oscillation model (GOAO). According to this model, the asymmetric double-peak emission lines are the result of the precession of a perturbation, in the form of a high-density zone, confined in the disc (Okazaki 1991, PASJ 43, 75; Papaloizou et al. 1992, A&A 265, L45). The order of the sequence is important since it provides the directional sense of the precession of the disc perturbation with respect to the Keplerian rotation. In the case of LS I +61 235, the V/R variations can only be explained if the perturbation revolves in the same direction as the stellar rotation, i.e. PROGRADE precession. Then, when the high-density part of the equatorial disk moves away from the observer the emission lines have V
In the movie we have represented the observed profile shape of the Halpha line in LS I+61 235 and a sketch of the global one-armed oscillation model, according to Telting et al. (1994, A&A 288, 558).