  The Mg II emission line is visible in the optical band for intermediate
redshift quasars (0.4 < z < 1.6) and it is thus an extremely important tool to
measure the black hole mass and to understand the structure of the Broad Line
Region. We aim to determine the substructure and the variability of the Mg II
line with the aim to identify which part of the line comes from a medium in
Keplerian motion. Using the Southern African Large Telescope (SALT) with the
Robert Stobie Spectrograph (RSS) we performed ten spectroscopic observations of
quasar HE 0435-4312 (z = 1.2231) over a period of three years (Dec 23/24, 2012
to Dec 7/8, 2015). Both the Mg II line and the Fe II pseudo-continuum increase
with time. We clearly detect the systematic shift of the Mg II line with
respect to the Fe II over the years, corresponding to the acceleration of 104
pm 14 km/s/year in the quasar rest frame. The Mg II line shape is clearly
non-Gaussian but single-component, and the increase in line equivalent width
and line shift is not accompanied with significant evolution of the line shape.
We analyse the conditions in the Mg II and Fe II formation region and we note
that the very large difference in the covering factor and the turbulent
velocity also support the conclusion that the two regions are spatially
separated. The measured acceleration of the line systematic shift is too large
to connect it with the orbital motion at a distance of the Broad Line Region
(BLR) in this source. It may imply a precessing inner disk illuminating the
BLR. Further monitoring is still needed to better constrain the variability
mechanism.
