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 3C 345 between 1989 and 1995, during which period the source flared. The multifrequency light curves are used to construct snap-shot spectra that span the synchrotron self-absorption turnover frequency. After subtraction of the underlying quiescent level, the flare spectrum is isolated and followed during the outburst. The evolution of the turnover frequency (ν_m_ with turnover flux (S_m_) is examined and compared with the predictions of the Marscher & Gear shocked-jet model. We find that 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 time. The extent of this steepening suggests that little reacceleration is occurring in the shock. We find that a power law is a remarkably good approximation to the observed trend of S_m_ with ν_m_. The turn over moves smoothly toward lower frequency with time, as expected from an emitting region that is expanding. Furthermore, this movement is observed to slow down with time, as predicted by the model. Initially, the turnover flux rises and then decays with a power-law index of ~1.0. This decay is interrupted by a second rise, which also decays with an index of ~1.0. It is found that the initial rise corresponds to the expansion phase, with the jet bending toward the line of sight rather than to the Compton or synchrotron phases of the model. Furthermore, the decays can only be reconciled with the model if the jet is assumed to be nonadiabatic or if it curves away from the line of sight during this period. The implied magnetic field orientation is parallel to the shock front, as expected from compression of the underlying 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 approximately perpendicular (B_parallel_) to the jet axis.