SALT long-slit spectroscopy of LBQS 2113-4538: variability of the Mg II
  and Fe II component
  The Mg II line is of extreme importance in intermediate redshift quasars
since it allows us to measure the black hole mass in these sources and to use
these sources as probes of the distribution of dark energy in the Universe, as
a complementary tool to SN Ia. Reliable use of Mg II requires a good
understanding of all the systematic effects involved in the measurement of the
line properties, including the contamination by Fe II UV emission. We performed
three spectroscopic observations of a quasar LBQS 2113-4538 (z = 0.956) with
the SALT telescope separated in time by several months and we analyze in detail
the mean spectrum and the variability in the spectral shape. We show that even
in our good-quality spectra the Mg II doublet is well fit by a single
Lorentzian shape. We tested several models of the Fe II pseudo-continuum and
showed that one of them well represents all the data. The amplitudes of both
components vary in time, but the shapes do not change significantly. The
measured line width of LBQS 2113-4538 identifies this object as a class A
quasar. The upper limit of $3\%$ for the contribution of the Narrow Line Region
(NLR) to Mg II may suggest that the separation of the Broad Line Region (BLR)
and NLR disappears in this class of objects.
