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Paper: Magnetic Field Structure in the Radio Core of BL Lac: Variable Core Rotation Measure and Detection of a Jet Boundary Layer
Volume: 340, Future Directions in High Resolution Astronomy: The 10th Anniversary of the VLBA
Page: 155
Authors: Mutel, L.R.; Denn, G.R.; Dreier, C.
Abstract: We have monitored the morphology and magnetic field structure of BL Lac's parsec-scale radio jet at nine epochs from 1998.7 - 2002.5 using the VLBA. The jet components all have low rotation measure (RM) and nearly perpendicular magnetic field structure at all epochs, consistent with transverse shocks and little foreground thermal gas. In contrast, the core has high variable RM, up to 8,200 rad-m-2 at some epochs, but the true RM may be underestimated by blending from emerging jet components. We also detected weak peripheral emission surrounding the main jet at some epochs with nearly parallel magnetic field and high fractional polarization. We suggest this may be a boundary layer caused by interaction with the ambient medium. Finally we searched for a 2.3 yr periodic variation in the structural position angle (SPA) of the innermost core component of BL Lac, as reported by Stirling et al. (2003). Although we found several possible periodicities, including one near the period reported by Stirling et al., our data are also consistent with no position angle variations.
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