| Abstract |
X-ray binaries are an important group of sources to study because they are among the brightest objects in the sky and are characterized by dramatic variability in brightness on timescales from milliseconds to months and years. Neutron-star X-ray binaries divide into high-mass (HMXB) and low-mass (LMXB) systems according to the mass of the donor star. High-mass X-ray binaries (> 8MΘ) divide further into supergiant X-ray binaries (SGXB) and Be/X-ray binaries (BeXB) depending on the evolutionary state of the optical companion. Be/X-ray binaries are primarily composed of a massive, early-type donor star (Oe or Be star) and a neutron star accretor. The rapidly rotating Be star is surrounded by a geometrically thin circum-stellar disk in its equator. The variability of the structure of the disk can be studied by observing the Balmer emission lines in the optical spectra and in particular the Ha emission line (6563A) which is the strongest and the best-studied of these lines. The interaction of the neutron star with the material in the disk results in transient X-ray behavior. Aim of this project is to perform a global study of the optical spectral properties of Be/X-ray binary systems. We analyzed the optical spectroscopic data for the optical counterparts of 14 BeXBs and study the long-term variability of the Hα emission line. In particular, we study the correlation between the X-ray activity and the strength of the Hα line, the long-term V/R ratio variations and the relationship between the disk size and the Hα line equivalent width. From the relationship between the peak separation and the equivalent width we confirm that the disks in BeXBs are Keplerian, as in the case of classical Be stars. However, the disks in BeXB are denser and smaller than those in isolated Be stars. Likewise, the characteristic timescales associated with the evolution of the disk are, on average, shorter in BeXB than in isolated Be stars.
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Study of the optical spectral variability of the Be/X-ray binary systems
