The Spectral Evolution of High-Frequency Radio Outbursts in the Blazar PKS:0420-014

Multifrequency monitoring data between 375 and 22 GHz are used to investigate spectral evolution of radio outbursts in the blazar 0420-014. It is shown that, after subtraction of the underlying `quiescent' emission, the flare spectrum can be described by a simple homogeneous self-absorbed synchrotron component. The evolution of the spectrum is found to be qualitatively consistent with the shock model of Marscher and Gear, although not totally so in detail. Some of the discrepancies can possibly be resolved if the proposed relativistic jet, which is initially oriented close to the line-of-sight, bends away. The optically thin spectrum of the flare emission, however, is found to be flatter than allowed by the model. Strong correlations are found between the 9~22 GHz two point spectral index and the logarithm of the 90-GHz flux. These trends are reconciled with the evolution of the synchrotron spectrum during the flares.