Name of the Speaker(s): Lourdes VICENTE
Institution: (DARC) Observatoire de Meudon
Title of the communication: Misalignment effects in BL Lac objects
Preliminary abstract (12 lines max.):
A large fraction of extragalactic radio sources show strong misalignment between the VLBI and VLA structure. From our sample of 155 objects, 37\% of the sources present an apparent misalignment larger than 45 deg and up to 71 \% for the subsample of BL LAc objects. The secondary peak near 90 deg in the apparent $\Delta PA$ distribution reported in litterature is difficult to fit with single population simple bend models, especially for BL Lac and intermediate redshift sources. This distribution is significantly different for BL Lacs and quasars. Consideration of different types of phenomena which might be responsible for misalignment lead us to suggest the existence of twisted accretion disks in the central engines of highly distorted radio sources (Appl,Sol and Vicente, 1996, A\&A 310, 419). Double jet models describe jets as a stratified structure with an inner relativistic beam surrounded by a slower collimated wind. In the context of such models, the presence of highly twisted accretion disks can lead to propagation of the inner jet through a transverse magnetic field. We study this scenario as a possible explanation for the particular magneticfield configuration observed in BL Lac objects, perpendicular to the VLBI jet (Sol and Vicente, 1996, preprint).