Helical Distortions of the Relativistic Jet in the BL Lacertae Object Mrk~501 (B1652+398)
Session 107 -- BL Lacs and Blazars
Display presentation, Saturday, January 15, 9:30-6:45, Salons I/II Room (Crystal Gateway)

## [107.05] Helical Distortions of the Relativistic Jet in the BL Lacertae Object Mrk~501 (B1652+398)

J.E. Conway and J.M. Wrobel (NRAO)

Mrk~501 belongs to a class of core--dominated radio sources with orthogonal misalignments between the position angles of their pc and kpc scale radio jets. Such geometries are expected if precessional or orbital motions in the nucleus drive Kelvin--Helmholtz instabilities in the jet, and these instabilities ultimately saturate (Conway \& Murphy 1993, ApJ, 411, 89; Hardee et al. 1994, ApJ, in press). Application of this model to new MERLIN and VLBA images of Mrk~501 supports (1) a driving period $P \sim 10^5$ yr; (2) an opening angle $2\zeta \sim 30^{\circ}$ for the cone defining the helical jet path before saturation; and (3) a bulk Lorentz factor $\gamma \sim 1.3$ for the fluid jet. Currently, values for $\zeta$ and $\gamma$ are strongly coupled but improved VLBA imaging can eliminate this coupling.