Centimeter-wavelength total flux and linear polarization properties of radio-loud BL Lacertae objects

We present results from a long-term program to quantify the range of behavi or of the centimeter-wavelength total flux and linear polarization variabil ity properties of a sample of 41 radio-loud BL Lac objects using weekly to trimonthly observations with the University of Michigan 26 m telescope oper ating at 14.5, 8.0, and 4.8 GHz; these observations are used to identify cl ass-dependent differences between these BL Lacs and QSOs in the Pearson-Rea dhead sample. As a group, the BL Lacs are found to be more highly variable in total flux density than the QSOs. These changes are often nearly simulta neous and of comparable amplitude at 14.5 and 4.8 GHz, which contrasts with the behavior in the QSOs and supports the existence of class-dependent dif ferences in opacity within the parsec-scale jet flows. Structure-function a nalyses of the flux observations quantify that a characteristic timescale i s identifiable in only one-third of the BL Lacs and that in the majority of the program sources the activity is uncorrelated within the timescales pro bed. The time-averaged fractional linear polarizations are only on the orde r 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 electric vector position angle is present. When compared with the very long baseline interferometry structural axis, no preferred p osition angle difference is identified; the derived distribution resembles that known for core components from very long baseline polarimetry measurem ents. The polarized flux typically exhibits variability with timescales of months to a few years and exhibits the signature of a propagating shock dur ing several resolved outbursts. The flux and polarization variability indic ate that the source emission is predominately due to evolving source compon ents and supports the occurrence of more frequent shock formation in BL Lac parsec-scale flows than in QSO jets, where the magnetic field topology eve n during outbursts is similar to that of the underlying quiescent Bow. The differences that we find in variability behavior and polarization between B L Lacs and QSOs can be explained by differences in stability between the je t flows found by recent studies of relativistic hydrodynamic flows.