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The Mg II emission line is visible in the optical band for intermediateredshift quasars (0.4 < z < 1.6) and it is thus an extremely important tool tomeasure the black hole mass and to understand the structure of the Broad LineRegion. We aim to determine the substructure and the variability of the Mg IIline with the aim to identify which part of the line comes from a medium inKeplerian motion. Using the Southern African Large Telescope (SALT) with theRobert Stobie Spectrograph (RSS) we performed ten spectroscopic observations ofquasar HE 0435-4312 (z = 1.2231) over a period of three years (Dec 23/24, 2012to Dec 7/8, 2015). Both the Mg II line and the Fe II pseudo-continuum increasewith time. We clearly detect the systematic shift of the Mg II line withrespect to the Fe II over the years, corresponding to the acceleration of 104pm 14 km/s/year in the quasar rest frame. The Mg II line shape is clearlynon-Gaussian but single-component, and the increase in line equivalent widthand 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 notethat the very large difference in the covering factor and the turbulentvelocity also support the conclusion that the two regions are spatiallyseparated. The measured acceleration of the line systematic shift is too largeto 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 theBLR. Further monitoring is still needed to better constrain the variabilitymechanism.

SALT long-slit spectroscopy of HE 0435-4312: fast displacement in the Mg II emission line