SALT long-slit spectroscopy of CTS C30.10: two-component Mg II line
  Quasars can be used as a complementary tool to SN Ia to probe the
distribution of dark energy in the Universe by measuring the time delay of the
emission line with respect to the continuum. The understanding of the Mg II
emission line structure is important for cosmological application and for the
black hole mass measurements of intermediate redshift quasars. The knowledge of
the shape of Mg II line and its variability allows to determine which part of
the line should be used to measure the time delay and the black hole mass. We
thus aim at determination of the structure and the variability of the Mg II
line as well as of the underlying Fe II pseudo-continuum. We performed five
spectroscopic observations of a quasar CTS C30.10 (z = 0.9000) with the SALT
telescope between December 2012 and March 2014, and we studied the variations
of the spectral shape in the 2700 - 2900 A rest frame. We showed that the Mg II
line in this source consists of two kinematic components, which makes the
source representative of type B quasars. Both components were well modeled with
a Lorentzian shape. Both components vary in a similar way. The Fe II
contribution seems to be related only to the first (blue) Mg II component}.
Broad band spectral fitting rather favor the use of the whole line profile. The
contribution of the Narrow Line Region to Mg II is very low, below 2%. The Mg
II variability is lower than the variability of the continuum, which is
consistent with the simple reprocessing scenario. The variability level of CTS
C30.10 and the measurement accuracy of the line and continuum is high enough to
expect that further monitoring will allow to measure the time delay between the
Mg II line and continuum.
