Studies of stationary features in jets: BL Lacertae. I. Dynamics and
  brightness asymmetry on sub-parsec scales
  Context. Monitoring of BL Lacertae at 15 GHz with the Very Long Baseline
Array (VLBA) has revealed a quasi-stationary radio feature in the innermost
part of the jet, at 0.26 mas from the radio core. Stationary features are found
in many blazars, but they have rarely been explored in detail. Aims. We aim to
study the kinematics, dynamics, and brightness of the quasi-stationary feature
of the jet in BL Lacertae based on VLBA monitoring with submilliarcsecond
resolution (subparsec-scales) over 17 years. Methods. We analysed position
uncertainties and flux leakage effects of the innermost quasi-stationary
feature and developed statistical tools to distinguish the motions of the
stationary feature and the radio core. We constructed a toy model to simulate
the observed emission of the quasi-stationary component. Results. We find that
trajectories of the quasi-stationary component are aligned along the jet axis,
which can be interpreted as evidence of the displacements of the radio core.
The intrinsic motions of the core and quasi-stationary component have a
commensurate contribution to the apparent motion of the stationary component.
During the jet-stable state, the core shift significantly influences the
apparent displacements of the stationary component, which shows orbiting motion
with reversals. The quasi-stationary component has low superluminal speeds on
time scales of months. On time-scales of few years, the apparent mean speeds
are subrelativistic, of about 0.15 the speed of light. We find that the
brightness profile of the quasi-stationary component is asymmetric along and
transverse to the jet axis, and this effect remains unchanged regardless of
epoch. Conclusions. Accurate positional determination, a high cadence of
observations, and a proper accounting for the core shift are crucial for the
measurement of the trajectories and speeds of the quasi-stationary component.
(abbrev.)
