Centimeter‐Wavelength Total Flux and Linear Polarization Properties of Radio‐loud BL Lacertae Objects

Abstract

We present 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-meter telescope operating at 14.5, 8.0, and 4.8 GHz; these observations are used to identify class-dependent differences between these BL Lacs and QSOs in the Pearson-Readhead 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-simultaneous 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 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 and that in the majority of the program sources the activity is uncorrelated within the timescales probed. 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 electric vector position angle is present. When compared with the VLBI structural axis, no preferred position angle difference is identified; the derived distribution resembles that known for core components from VLBP measurements. The polarized flux typically exhibits variability with timescales of months to a few years and exhibits the signature of a propagating shock during several resolved outbursts. The flux and polarization variability indicate that the source emission is predominately due to evolving source components and supports the occurrence of more frequent shock formation in BL Lac parsec-scale flows than in QSO jets where the magnetic field topology -3even during outbursts is similar to that of the underlying quiescent flow. The differences in variability behavior and polarization between BL Lacs and QSOs which we find can be explained by differences in stability between the jet flows found by recent studies of relativistic hydrodynamic flows.

Core shift effect in blazars

“…The long term light curves of blazars are obtained from the University of Michigan Radio Astronomical Observatory (UMRAO) which has monitored compact variable radio sources spanning over ∼ 40 years with a 26 m paraboloid dish and provides multifrequency (4.8, 8.0 and 14.5 GHz), high time resolution (better than a month) light curves (Aller et al 1985(Aller et al , 1999 (Nesterov, Volvach, & Strepka 2000). The 14 m radio telescope of Aalto University Metsähovi Radio Observatory in Finland was used for observations at 37.0 GHz.…”

The kinematics in the pc-scale jets of AGN

“…Such time-delayed outbursts are associated observationally on mas-scales with the brightening of the VLBI core and are accompanied by the ejection of a new optically thin jet component. We decomposed the total flux-density light-curves at 4.8 GHz, 8 GHz, and 14.5 GHz from the University of Michigan Radio Observatory (UMRAO) monitoring campaign (Aller et al 1999 into Gaussian components, as was described in Pyatunina et al (2006Pyatunina et al ( , 2007. Figure 3 shows the light-curve of S5 1803+784 at 14.5 GHz and the resulting Gaussian decomposition.…”

“…The components follow We investigated possible correlations among the jetcomponent parameters and the total flux-density variability. For this purpose we used the UMRAO data of multi-frequency AGN monitoring (Aller et al 1999, spanning almost 30 years (see Fig. 11).…”

A multifrequency analysis of radio variability of blazars

“…The UMRAO group have monitored and analyzed the light curves of two complete samples, the Pearson-Readhead sample ) and a BL Lac sample (Aller et al 1999). …”