High-dispersion spectroscopic monitoring of the Be/X-ray binary A0535+26/V725 Tau - I. The long-term profile variability
We report on optical high-dispersion spectroscopic monitoring observations of the Be/X-ray binary A0535+26/V725 Tau, carried out from 2005 November to 2009 March. The main aim of these monitoring observations is to study spectral variabilities in the Be disc, on both the short (a week or so) and long (more than hundreds of days) time-scales, by taking long-term frequent observations. Our four-year spectroscopic observations indicate that the V/R ratio, i.e. the relative intensity of the violet (V) peak with respect to the red (R) one, of the double-peaked Halpha line profile varies with a period of 500d. The Hbeta line profile also varies in phase with the Halpha profile. With these observations covering two full cycles of the V/R variability, we reconstruct the two-dimensional structure of the Be disc by applying the Doppler tomography method to the Halpha and Hbeta emission-line profiles, using a rigidly rotating frame with the V/R variability period. The resulting disc structure reveals non-axisymmetric features, which can be explained by a one-armed perturbation in the Be disc. It is the first time that an eccentric disc structure is directly detected by using a method other than the interferometric one.