The Doppler crisis in TeV blazars and its possible resolutions

Different observers have estimated drastically different bulk Lorentz factors and Doppler factors for the jets of the TeV blazars, depending on the method used. On the one hand, rapid variability at high energies on timescales of a few minutes and fits to the spectral energy distributions (SEDs) imply large Doppler factors approaching 100. On the other hand, jet morphologies, kinematics, and brightness temperatures measured from parsec-scale imaging with the Very Long Baseline Array (VLBA) imply at most modest Doppler boosting. Such modest Doppler boosting is also suggested by unification arguments. This so-called Doppler crisis may indicate the presence of structured jets in these sources, with different regions of different Lorentz factors, for example, a fast spine and a slower layer. We discuss the constraints on such structured jet models from our latest observations of these sources with the VLBA