AAS 202nd Meeting, May 2003
Session 18 Blazars and Radio Sources
Oral, Monday, May 26, 2003, 2:00-3:30pm, 204

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[18.04] Magnetic Field Structure in the Radio Core of BL Lac: Variable Core RM and Detection of a Jet Boundary Layer

R. L. Mutel (University of Iowa), G. R. Denn (Sweet Briar College)

We report multi-frequency VLBP monitoring of parsec-scale radio core of the AGN BL Lac over nine epochs from 1998.7 to 2002.0. We find that all jet components have nearly perpendicular magnetic field orientation and low rotation measure (RM) at all epochs, consistent with shocked-jet models. We confirm the expected trajectory and speed of superluminal component S10 as predicted by the helical model parameters of Denn et al. (2000). A new jet component (S11) has appeared within the jet with no evidence for evolution from the core, although this may be a result of variable Doppler boosting on a curved trajectory.

The core exhibits highly variable RM, ranging from <500 rad m-2 to ~5000 rad m-2. The fractional polarization of the core is wavelength-dependent, with an approximate scaling p(\lambda) \alpha \lambda-2. Both the variable RM and the fractional polarization may be modeled as a result of a turbulent foreground screen with characteristic RM fluctuations \sigmaRM~4000 rad m-2 and a scale size 0.01-0.03 pc.

At several epochs we detect a weak sheath region with magnetic field orientation nearly parallel to the jet direction, suggesting a boundary interaction layer. This is similar to the longitudinal field component previously reported for the parsec-scale jet in the blazar 1055+018 (Attridge et al. 1999) and several kpc-scale AGN jets.

The author(s) of this abstract have provided an email address for comments about the abstract: robert-mutel@uiowa.edu

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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.