The University of Michigan radio astronomy data base. I - Structure function analysis and the relation between BL Lacertae objects and quasi-stellar objects

Abstract

We have performed a structure function analysis on a well-observed group of variable extragalactic sources, monitored in total and polarized flux in the centimeter waveband since 1965. Total flux structure functions of BL Lac objects and QSOs exhibit very similar power-law slopes, suggesting that the same processes are responsible for the variability in both types of source. In view of the recent success of the shocked-jet model for radio source variability, we argue that the current work provides circumstantial evidence for the ubiquity of such shocked flows. This analysis has provided a good estimate of characteristic time scales of variability for the two types of object: there is a very broad distribution of time scales for both BL Lac objects and QSOs with mean time scales of 1.95 and 2.35 yr, respectively. The majority of sources (>~85%) with time scales greater than 10 yr are QSOs. A structure function analysis applied to the Stokes parameters in a frame of reference related to the VLBI jet structure suggests that variations of the polarized flux occur in an orientation related to the underlying jet direction. This is further circumstantial evidence for the domination of the source polarization by a shock phenomenon, as this behavior is what one would expect from the axial compression of a turbulent magnetic field. We interpret our derived time scales for Q and U variations and the relative amplitude of these variations, in terms of emission from shocked jets with a well-defined magnetic field direction, contaminated by a significant but randomly polarized core emission for BL Lacs, and in similar terms, but with a magnetic field less strongly correlated with the jet direction, and a weakly polarized core, for QSOs. Periodogram analyses show little or no evidence of harmonic behavior. Subject Headings BL Lacertae objects: general - galaxies: jets - polarization - quasars: general - radio continuum: galaxies