Abstract:
We present results of the VLBA3 polarization observations of
"X-ray selected" BL Lac objects from the HEAO-l survey and also a few from
the ROSAT-Green Bank (RGB) sample. These primarily high energy peaked
BL Lacs (HBLs) have "core-jet" morphologies with modestly polarized pc-scale
cores and jet components, similar to their radio-selected counterparts (primarily
low energy peaked BL Lacs, LBLs). However, we find that the jets in HBLs
show preferentially longitudinal magnetic fields, as opposed to the jets in LBLs,
which tend to have transverse magnetic fields.
