The extreme blazar AO 0235+164 as seen by extensive ground and space
  radio observations
  Clues to the physical conditions in radio cores of blazars come from
measurements of brightness temperatures as well as effects produced by
intrinsic opacity. We study the properties of the ultra compact blazar AO
0235+164 with RadioAstron ground-space radio interferometer, multi-frequency
VLBA, EVN and single-dish radio observations. We employ visibility modeling and
image stacking for deriving structure and kinematics of the source, and use
Gaussian process regression to find the relative multi-band time delays of the
flares. The multi-frequency core size and time lags support prevailing
synchrotron self absorption. The intrinsic brightness temperature of the core
derived from ground-based VLBI is close to the equipartition regime value. In
the same time, there is evidence for ultra-compact features of the size of less
than 10 $\mu$as in the source, which might be responsible for the extreme
apparent brightness temperatures of up to $10^{14}$ K as measured by
RadioAstron. In 2007--2016 the VLBI components in the source at 43 GHz are
found predominantly in two directions, suggesting a bend of the outflow from
southern to northern direction. The apparent opening angle of the jet seen in
the stacked image at 43 GHz is two times wider than that at 15 GHz, indicating
a collimation of the flow within the central 1.5 mas. We estimate the Lorentz
factor $\Gamma = 14$, the Doppler factor $\delta=21$, and the viewing angle
$\theta = 1.7^\circ$ of the apparent jet base, derive the gradients of magnetic
field strength and electron density in the outflow, and the distance between
jet apex and the core at each frequency.
