How circumstellar discs affect mass accretion in Be X-ray binaries

Includes bibliographical referencesLong-term optical spectroscopic monitoring of Galactic Be X-ray binaries (BeXBs) is performed using the Liverpool Telescope and the Southern African Large Telescope on northern and southern objects, respectively. Be disc size variations are presented to investigate their influence on mass accretion producing X-ray activity. Be disc variability is traced observationally through Balmer emission lines, the strongest and best studied being the Hα line. The peak separations of the double peaked Hα emission line are measured, and along with the mass of the Be star and the inclination of the disc obtained from literature, are used to determine the Be disc radius. For single peaked Hα profiles the peak separation cannot be obtained directly; however, using the empirically determined relationship between the equivalent width and the peak separation of the double peaked profiles, an estimate of the peak separation for the single peaked profiles is obtained. The work is done in the context of the viscous decretion disc model presented by Okazaki & Negueruela (2001), which predicts that the circumstellar discs around Be stars in binary systems are truncated by resonant torques from the neutron star in its orbit. The calculated disc radii are compared to the expected resonance radii from the viscous decretion disc model to determine how different truncation radii affect mass accretion producing X-ray outbursts. Type I outbursts are seen to occur when the disc is truncated close to/beyond the mean critical Roche lobe radius at periastron passage of the neutron star, in agreement with model predictions. Type II outbursts, however, do not show any correlation (or anticorrelation) with the disc size, as they are seen to occur both at relatively small and large sizes of the disc. Additional information on the Hα emission line profile variations, such as the line-shape variations of high-resolution spectra, is required to investigate the origin of type II outbursts in order to make reliable predictions of them. Spectroscopic follow-up of the gamma-ray binary system LSI +61 303 was also performed with the Liverpool Telescope and the Hα line profile variations are presented. In addition to data from the Liverpool Telescope monitoring campaign, published equivalent width measurements are used and timing analysis of the combined measurements is performed. The orbital period and super-orbital period are found, which are similar to those found at other wavelengths. The periodicities in the system can therefore be associated with circumstellar disc variations