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Paper: Survey of Variability
Volume: 300, Radio Astronomy at the Fringe
Page: 159
Authors: Aller, M.F.; Aller, A.D.; Hughes, P.A.
Abstract: Centimeter-band total flux density and linear polarization variability is a common property of extragalactic objects, including many with a steep spectrum: these variations occur on a variety of timescales and, in several cases, are clearly associated with the development of VLBI-scale hot spots. Within the framework of a hydrodynamical relativistic flow model we attribute these variations to the development of weak shocks in the parsec-scale flows; these arise from naturally-developing Kelvin-Helmholtz instabilities. The observed changes in electric vector position angle (EVPA) and degree of polarization during well-resolved events are consistent with a scenario in which an initially-tangled magnetic field becomes more ordered with the passage of a shock. Differences in magnetic field orientation relative to flow direction as a function of optical classification are discussed. Dual-wavelength observations of circular polarization suggest that variations in circular polarization (CP) are produced by linear-to-circular mode conversion in a partially opaque region of the source, while well-documented changes in polarity in the Stokes parameter, V, demonstrate that the magnetic field does not retain its handedness over time. Combined X-ray/radio-band monitoring of 3C 120 provides evidence for causally-related activity, plausibly yielding the first connection between activity in the radio jet and the central engine.
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