Wavelength-resolved Reverberation Mapping of quasar CTSC30.10:
  Dissecting MgII and FeII emission regions
  We present the results of the reverberation monitoring aimed at MgII broad
line and FeII pseudocontinuum for the luminous quasar CTS C30.10 (z = 0.90052)
with the Southern African Large Telescope covering the years 2012-2021. We
aimed at disentangling the MgII and UV FeII variability and the first
measurement of UV FeII time delay for a distant quasar. We used several methods
for time-delay measurements and determined both FeII and MgII time delays as
well as performed a wavelength-resolved time delay study for a combination of
MgII and FeII in the 2700 - 2900 \AA restframe wavelength range. We obtain the
time delay for MgII of $275.5^{+12.4}_{-19.5}$ days in the rest frame, while
for FeII we have two possible solutions of $270.0^{+13.8}_{-25.3}$ days and
$180.3^{+26.6}_{-30.0}$ in the rest frame. Combining this result with the old
measurement of FeII UV time delay for NGC 5548 we discuss for the first time
the radius-luminosity relation for UV FeII with the slope consistent with $0.5$
within uncertainties. Since FeII time delay has a shorter time-delay component
but lines are narrower than MgII, we propose that the line delay measurement is
biased towards the BLR part facing the observer, with the bulk of the Fe II
emission may arise from the more distant BLR region, one that is shielded from
the observer.
