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Inverse Compton cooling limits the brightness temperature of the radiatingplasma to a maximum of $10^{11.5}$ K. Relativistic boosting can increase itsobserved value, but apparent brightness temperatures much in excess of$10^{13}$ K are inaccessible using ground-based very long baselineinterferometry (VLBI) at any wavelength. We present observations of the quasar3C273, made with the space VLBI mission RadioAstron on baselines up to 171,000km, which directly reveal the presence of angular structure as small as 26$\mu$as (2.7 light months) and brightness temperature in excess of $10^{13}$ K.These measurements challenge our understanding of the non-thermal continuumemission in the vicinity of supermassive black holes and require a much higherDoppler factor than what is determined from jet apparent kinematics.

RadioAstron Observations of the Quasar 3C273: a Challenge to the Brightness Temperature Limit