Multi-Epoch VLBA Imaging of Twenty New TeV Blazars: Apparent Jet Speeds
  We present 88 multi-epoch Very Long Baseline Array (VLBA) images (most at an
observing frequency of 8 GHz), of 20 TeV blazars, all of the HBL class, that
have not been previously studied at multiple epochs on the parsec scale. From
these 20 sources, we analyze the apparent speeds of 43 jet components that are
all detected at four or more epochs. As has been found for other TeV HBLs, the
apparent speeds of these components are relatively slow. About two-thirds of
the components have an apparent speed that is consistent (within two-sigma)
with no motion; and some of these components may be stationary patterns whose
apparent speed does not relate to the underlying bulk flow speed. In addition,
a superluminal tail to the apparent speed distribution of the TeV HBLs is
detected for the first time, with eight components in seven sources having a
two-sigma lower limit on the apparent speed exceeding 1c. We combine the data
from these 20 sources with an additional 18 sources from the literature to
analyze the complete apparent speed distribution of all 38 TeV HBLs that have
been studied with VLBI at multiple epochs. The highest two-sigma apparent speed
lower limit considering all sources is 3.6c. This suggests that bulk Lorentz
factors of up to about 4, but probably not much higher, exist in the
parsec-scale radio emitting regions of these sources, consistent with estimates
obtained in the radio by other means such as brightness temperatures. This can
be reconciled with the high Lorentz factors estimated from the high-energy data
if the jet has velocity structures consisting of different emission regions
with different Lorentz factors. In particular, we analyze the current apparent
speed data for the TeV HBLs in the context of a model with a fast central spine
and a slower outer layer.
