Evidence for parsec-scale absorption from VSOP observations of the BL Lacertae object 0735+178

VLBI total intensity and linear polarization images of the BL Lacertae object 0735+178 have been obtained at λ=6cm using the VLBA+phased VLA as a ground array together with the HALCA orbiting antenna, and at λ=2cm about a month later using the VLBA. The most surprising result to come from a comparison of these two images, which have nearly equivalent resolution, is that the path of the jet appears to be appreciably different at the two wavelengths. We have interpreted this as evidence for free-free absorption to the east of the core, near a bend of about 90° in the 2-cm image where the emission is very weak in the 6-cm image. Overall, the polarization structures seen in corresponding regions at the two wavelengths are in good agreement. We find tentative evidence for enhanced rotation measures near two sharp bends in the VLBI jet.