RadioAstron Observations of the Quasar 3C273: a Challenge to the
  Brightness Temperature Limit
  Inverse Compton cooling limits the brightness temperature of the radiating
plasma to a maximum of $10^{11.5}$ K. Relativistic boosting can increase its
observed value, but apparent brightness temperatures much in excess of
$10^{13}$ K are inaccessible using ground-based very long baseline
interferometry (VLBI) at any wavelength. We present observations of the quasar
3C273, made with the space VLBI mission RadioAstron on baselines up to 171,000
km, 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 continuum
emission in the vicinity of supermassive black holes and require a much higher
Doppler factor than what is determined from jet apparent kinematics.
