  The optical behaviour of the Be star in the high-mass X-ray transient
A0535+26/HDE245770 shows that at periastron the luminosity is typically
enhanced by 0.02 to a few tenths magnitude, and the X-ray outburst occurs eight
days after the periastron. Indeed, at the periastron an increase of the mass
flux occurs. This sort of flush reaches the external part of the temporary
accretion disk around the neutron star and moves to the hot central parts of
the accretion disk and the neutron star surface. The time necessary for this
way is dependent on the turbulent viscosity in the accretion disk, as discussed
by Giovannelli, Bisnovatyi-Kogan, and Klepnev (2013) (GBK13). In this paper we
will show the behaviour of this system in optical band around the predicted
periastron passage on 21st February 2014, by using the GBK13 ephemeris that we
used to schedule our spectroscopic and photometric optical observations.
Spectroscopic unusual activity detected in the Balmer lines and the enhancement
in the emission in B, V, and R bands around the periastron passage, and the
subsequent X-ray event definitively demonstrate the existence of about 8 day
delay between optical and X-ray flares.
