2021MNRAS.504.5074S

The results of a long-term multiwavelength study of the powerful flat spectrum radio quasar 3C 454.3 using Fermi-LAT and Swift XRT/UVOT data are reported. In the γ-ray band, Fermi-LAT observations show several major flares when the source flux was > 10^–5 photon cm^–2 s^–1; the peak γ-ray flux above 141.6 MeV, (9.22±1.96)×10^–5 photon cm^–2 s^–1 observed on MJD 55519.33, corresponds to 2.15×10⁵⁰ erg s^–1 isotropic γ-ray luminosity. The analysis of Swift XRT and UVOT data revealed a flux increase, although with smaller amplitudes, also in the X-ray and optical/UV bands. The X-ray emission of 3C 454.3 is with a hard spectral index of Γ_X = 1.16-1.75, and the flux in the flaring states increased up to (1.80±0.18)×10^–10 erg cm^–2 s^–1. Through combining the analysed data, it was possible to assemble 362 high-quality and quasi-simultaneous spectral energy distributions of 3C 454.3 in 2008-2018, which all were modelled within a one-zone leptonic scenario assuming the emission region is within the broad-line region, involving synchrotron, synchrotron self-Compton, and external Compton mechanisms. Such an extensive modelling is the key for constraining the underlying emission mechanisms in the 3C 454.3 jet and allows to derive the physical parameters of the jet and investigate their evolution in time. The modelling suggests that during the flares, along with the variation of emitting electron parameters, the Doppler boosting factor increased substantially, implying that the emission in these periods has most likely originated in a faster moving region.