Flare-like Variability of the Mg II λ2798 Å Emission Line and UV Fe II Band in the Blazar CTA 102

We report on the detection of a statistically significant flare-like event in the Mg II λ 2798 Å emission line and the UV Fe II band of CTA 102 during the outburst of fall 2017. The ratio between the maximum and minimum of λ3000 Å continuum flux for the observation period (2010-2017) is 179 ± 15. Respectively, the max/min ratios 8.1 ± 10.5 and 34.0 ± 45.5 confirmed the variability of the Mg II emission line and of the Fe II band. The highest levels of emission line fluxes recorded coincide with a superluminal jet component traversing through a stationary component located ∼0.1 mas from the 43 GHz core. Additionally, comparing the Mg II line profile in the minimum of activity against the one in the maximum, we found that the latter is broader and blueshifted. As a result of these findings, we can conclude that the non-thermal continuum emission produced by material in the jet moving at relativistic speeds is related to the broad emission line fluctuations. Consequently, these fluctuations are also linked to the presence of broad-line region (BLR) clouds located ∼25 pc from the central engine, outside the inner parsec, where the canonical BLR is located. Our results suggest that during strong activity in CTA 102, the source of non-thermal emission and broad-line clouds outside the inner parsec introduces uncertainties in the estimates of black hole (BH) mass. Therefore, it is important to estimate the BH mass, using single-epoch or reverberation mapping techniques, only with spectra where the continuum luminosity is dominated by the accretion disk.