THE EVOLUTION OF THE CENTIMETER-SUBMILLIMETER SPECTRUM OF 3C-345 DURING OUTBURST

Monitoring data between 375 and 4.8 GHz are presented for the blazar 3 C 345 between 1989 and 1995, during which period the source hared. The multifrequency Light curves are used to construct snapshot spectra th at span the synchrotron seif-absorption turnover frequency. After subt raction of the underlying quiescent level, the flare spectrum is isola ted and followed during the outburst. The evolution of the turnover fr equency (nu(m)) with turnover flux (S-m) is examined and compared with the predictions of the Marscher & Gear shocked-jet model. We find tha t the flare spectrum is well fitted by a homogeneous synchrotron curve for at least 2 yr after the initial rise in flux. The high-frequency optically thin section of this spectrum is observed to steepen with ti me. The extent of this steepening suggests that little reacceleration is occurring in the shock. We find that a power law is a remarkably go od approximation to the observed trend of S-m with nu(m). The turn ove r moves smoothly toward lower frequency with time, as expected from an emitting region that is expanding. Furthermore, this movement is obse rved to slow down with time, as predicted by the model. Initially, the turnover flux rises and then decays with a power-law index of similar to 1.0. This decay is interrupted by a second rise, which also decays with an index of similar to 1.0. It is found that the initial rise co rresponds to the expansion phase, with the jet bending toward the line of sight rather than to the Compton or synchrotron phases of the mode l. Furthermore, the decays can only be reconciled with the model if th e jet is assumed to be nonadiabatic or if it curves away from the line of sight during this period. The implied magnetic held orientation is parallel to the shock front, as expected from compression of the unde rlying field. This finding is supported by the total polarization data that suggest that the outbursts depolarize the total emission, which, during quiescent periods, is significantly polarized in a direction a pproximately perpendicular (B-parallel to) to the jet axis.