摘要：Aims.Characterisation of the long-term variations in the broad line region in a luminous blazar, where Comptonisation of broad-line emission within a relativistic jet is the standard scenario for production ofγ-ray emission that dominates the spectral energy distribution.Methods.We analysed ten years of optical spectroscopic data from the Steward Observatory for the blazar 3C 454.3, as well asγ-ray data from theFermiLarge Area Telescope (LAT). The optical spectra are dominated by a highly variable non-thermal synchrotron continuum with a prominent Mg II broad emission line. The line flux was obtained by spectral decomposition including significant contribution from the Fe II pseudo-continuum. Three methods were used to characterise variations in the line flux: (1) stacking of the continuum-subtracted spectra, (2) subtracting the running mean light curves calculated for different timescales, and (3) evaluating potential time delays via the discrete correlation function (DCF).Results.Despite very large variations in theγ-ray and optical continua, the line flux changes only moderately (<0.1 dex). The data suggest that the line flux responds to a dramatic change in the blazar activity from a very high state in 2010 to a deep low state in 2012. Two interpretations are possible: either the line flux is anti-correlated with the continuum or the increase in the line luminosity is delayed by ∼600 days. If this time delay results from the reverberation of poorly constrained accretion disc emission in both the broad-line region (BLR) and the synchrotron emitting blazar zone within a relativistic jet, we would obtain natural estimates for the BLR radiusRBLR, MgII ≳ 0.28 pc and for the supermassive black hole massMSMBH ∼ 8.5 × 108 M⊙. We did not identify additional examples of short-term “flares” of the line flux, in addition to the previously reported case observed in 2010.
