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Jets of compact radio sources are highly relativistic and Doppler boosted,making studies of their intrinsic properties difficult. Observed brightnesstemperatures can be used to study the intrinsic physical properties of therelativistic jets, and constrain models of jet formation in the inner jetregion. We aim to observationally test such inner jet models. The very longbaseline interferometry (VLBI) cores of compact radio sources are opticallythick at a given frequency. The distance of the core from the central engine isinversely proportional to the frequency. Under the equipartition conditionbetween the magnetic field energy and particle energy densities, the absolutedistance of the VLBI core can be predicted. We compiled the brightnesstemperatures of VLBI cores at various radio frequencies of 2, 8, 15, and86~GHz. We derive the brightness temperature on sub-parsec scales in the restframe of the compact radio sources. We find that the brightness temperatureincreases with increasing distance from the central engine, indicating that theintrinsic jet speed (the Lorentz-factor) increases along the jet. This impliesthat the jets are accelerated in the (sub-)parsec regions from the centralengine.

Acceleration of Compact Radio Jets on Sub-parsec Scales