The optical behaviour of the Be star in the high mass X-ray transient A0535+26/HDE245770 shows that at the periastron typically there is an enhancement in the luminosity of order 0.05 to few tenths mag , and the X- ray outburst happens about 8 days after the periastron .
We construct a quantitative model of this event , basing on a nonstationary accretion disk behavior , connected with a high ellipticity of the orbital motion .
We explain the observed time delay between the peaks of the optical and X-ray outbursts in this system by the time of radial motion of a matter in the accretion disk , after increase of the mass flux in the vicinity of a periastral point in the binary .
This time is determined by the turbulent viscosity , with the parameter \alpha = 0.1 - 0.3 , estimated from the comparison of the model with observational data