Synchrotron Emission from Shocked Relativistic Jets. II. A Model for the Centimeter Wave Band Quiescent and Burst Emission from BL Lacertae

We present a model for the radio emission from BL Lac based on a truncated, conical flow of synchrotron plasma with a tangled magnetic field. Outbursts are due to shocks that locally compress the field, allowing for a significant degree of polarized emission from the shocked part of the flow. We show that the shape of the total flux versus time curves, and the percentage polarization, independently suggest that we are viewing the source at an angle ~40^deg^ to the flow axis. Although the flow is highly relativistic, because of the large viewing angle the Doppler factor is of order unity and the apparent expansion speed is ~2c; this speed is compatible with VLBI observations if H_0_ ~ 100 km s^-1^ Mpc^-1^. We estimate that the magnetic field in the radio-emitting region is significantly below its energy equipartition value, and that the plasma is a high-pressure, freely expanding flow with a mechanical luminosity comparable to the bolometric luminosities of the most active AGNs.