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Clues to the physical conditions in radio cores of blazars come frommeasurements of brightness temperatures as well as effects produced byintrinsic opacity. We study the properties of the ultra compact blazar AO0235+164 with RadioAstron ground-space radio interferometer, multi-frequencyVLBA, EVN and single-dish radio observations. We employ visibility modeling andimage stacking for deriving structure and kinematics of the source, and useGaussian process regression to find the relative multi-band time delays of theflares. The multi-frequency core size and time lags support prevailingsynchrotron self absorption. The intrinsic brightness temperature of the corederived from ground-based VLBI is close to the equipartition regime value. Inthe same time, there is evidence for ultra-compact features of the size of lessthan 10 $\mu$as in the source, which might be responsible for the extremeapparent brightness temperatures of up to $10^{14}$ K as measured byRadioAstron. In 2007--2016 the VLBI components in the source at 43 GHz arefound predominantly in two directions, suggesting a bend of the outflow fromsouthern to northern direction. The apparent opening angle of the jet seen inthe stacked image at 43 GHz is two times wider than that at 15 GHz, indicatinga collimation of the flow within the central 1.5 mas. We estimate the Lorentzfactor $\Gamma = 14$, the Doppler factor $\delta=21$, and the viewing angle$\theta = 1.7^\circ$ of the apparent jet base, derive the gradients of magneticfield strength and electron density in the outflow, and the distance betweenjet apex and the core at each frequency.

The extreme blazar AO 0235+164 as seen by extensive ground and space radio observations