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M.F. Aller, H.D. Aller, P.A. Hughes (U. Mich.)
Statistical results, based on longterm observations of 2 flux-limited samples acquired to investigate the radioband flux and polarization variability properties of extragalactic objects are presented; these results are based on up to three decades of flux and polarization observations of 41 BL Lacs (UMRAO BL Lac sample) and 28 QSOs (Pearson-Readhead VLBI sample) obtained with the UMRAO 26-meter paraboloid, and published VLBI/P observations. A first-order structure function analysis of the flux variability shows that the activity is well-correlated in less than half of the sources. The mean for well-determined characteristic timescales \tau (in the source frame) is 1.54 years and is shortest, 22 days, for the intraday variable QSO~0804+499. The distribution of \tau quantifies the need for VLBI observations sampled at ~ monthly intervals for studying evolving source structure in the most active sources. In spite of the high degrees of variability in the polarized flux, associated with propagating transverse shocks in some cases, long-term preferred intrinsic polarization position angle orientations have been identified from peaks in histograms of monthly-averaged Stokes parameters corrected for Faraday rotation. These EVPAs are compared with VLBI/P structural axes, providing information about the orientation of the dominant magnetic field direction relative to the flow direction. Statistically, no preferred orientation difference for the BL Lacs is found; this result mimics that found for core components in VLBP studies and suggests that the integrated polarizations may be dominated by emission from unresolved cores. In the QSOs, longer term stability with a tendency for the magnetic field to align near to the flow direction (60\arcdeg\leq|EVPA-\Theta|\leq90\arcdeg) is identified; this is consistent with emission from a partially-ordered field with shear or, alternatively, from shocks oriented at oblique angles to the flow direction. Evidence in support of the common development of oblique shocks in relativistic flows is provided by recent 3-D numerical simulations.
This work was partially funded by a series of grants from the NSF.