Double SSA spectrum and magnetic field strength of the FSRQ 3C 454.3

We present the results of a radio multifrequency (3−340 GHz) study of the blazar 3C 454.3. After subtracting the quiescent spectrum corresponding to optically thin emission, we found two individual synchrotron self-absorption (SSA) features in the wide-band spectrum. The one SSA had a relatively low turnover frequency (ν_m) in the range of 3−37 GHz (lower ν_m SSA spectrum, LSS), and the other one had a relatively high ν_m of 55−124 GHz (higher ν_m SSA spectrum, HSS). Using the SSA parameters, we estimated B-field strengths at the surface where optical depth τ = 1. The estimated B-field strengths were > 7 and > 0.2 mG for the LSS and HSS, respectively. The LSS-emitting region was magnetically dominated before the 2014 June γ -ray flare. The quasi-stationary component (C), ∼0.6 mas apart from the 43 -GHz radio core, became brighter than the core with decreasing observing frequency, and we found that component C was related to the LSS. A decrease in jet width was found near component C. As a moving component, K14 approached component C, and the flux density of the component was enhanced while the angular size decreased. The high intrinsic brightness temperature in the fluid frame was obtained as T_{B, int} ≈ (7.0 ± 1.0) × 10¹¹ K from the jet component after the 2015 August γ -ray flare, suggesting that component C is a high-energy emitting region. The observed local minimum of jet width and re-brightening behaviour suggest a possible recollimation shock in component C.