Cm-Wavelength Total Flux and Linear Polarization Properties of   Radio-Loud BL Lacertae Objects
  Results from a long-term program to quantify the range of behavior of the cm-wavelength total flux and linear polarization variability properties of a sample of 41 radio-loud BL Lac objects using weekly to tri-monthly observations with the University of Michigan 26-m telescope operating at 14.5, 8.0, and 4.8 GHz are presented; these observations are used to identify class-dependent differences between these BL Lacs and QSOs in the Pearson-Readhead sample. The BL Lacs are found to be more highly variable in total flux density than the QSOs, exhibiting changes that are often nearly-simultaneous and of comparable amplitude at 14.5 and 4.8 GHz in contrast to the behavior in the QSOs and supporting the existence of class-dependent differences in opacity within the parsec-scale jet flows. Structure function analyses of the flux observations quantify that a characteristic timescale is identifiable in only 1/3 of the BL Lacs. The time-averaged fractional linear polarizations are only on the order of a few percent and are consistent with the presence of tangled magnetic fields within the emitting regions. In many sources a preferred long-term orientation of the EVPA is present; when compared with the VLBI structural axis, no preferred position angle difference is identified. The polarized flux typically exhibits variability with timescales of months to a few years and shows the signature of a propagating shock during several resolved outbursts. The observations indicate that the source emission is predominately due to evolving source components and support the occurrence of more frequent shock formation in BL Lac parsec-scale flows than in QSO jets. The differences in variability behavior and polarization between BL Lacs and QSOs can be explained by differences in jet stability. 
Margo F. Aller, Hugh D. Aller, Philip A. Hughes, and George E. Latimer