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

Abstract

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 (nu(m)) in the range of 3-37 GHz (lower nu(m) SSA spectrum, LSS), and the other one had a relatively high nu(m) of 55-124 GHz (higher nu(m) SSA spectrum, HSS). Using the SSA parameters, we estimated B-field strengths at the surface where optical depth tau = 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 gamma-ray flare. The quasi-stationary component (C), similar to 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,T- int approximate to (7.0 +/- 1.0) x 10(11) K from the jet component after the 2015 August.-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.