Probing the innermost regions of AGN jets and their magnetic fields with
  RadioAstron II. Observations of 3C 273 at minimum activity
  RadioAstron is a 10 m orbiting radio telescope mounted on the Spektr-R
satellite, launched in 2011, performing Space Very Long Baseline Interferometry
(SVLBI) observations supported by a global ground array of radio telescopes.
With an apogee of about 350 000 km, it is offering for the first time the
possibility to perform {\mu}as-resolution imaging in the cm-band. We present
observations at 22 GHz of 3C 273, performed in 2014, designed to reach a
maximum baseline of approximately nine Earth diameters. Reaching an angular
resolution of 0.3 mas, we study a particularly low-activity state of the
source, and estimate the nuclear region brightness temperature, comparing with
the extreme one detected one year before during the RadioAstron early science
period. We also make use of the VLBA-BU-BLAZAR survey data, at 43 GHz, to study
the kinematics of the jet in a 1.5-year time window. We find that the nuclear
brightness temperature is two orders of magnitude lower than the exceptionally
high value detected in 2013 with RadioAstron at the same frequency (1.4x10^13
K, source-frame), and even one order of magnitude lower than the equipartition
value. The kinematics analysis at 43 GHz shows that a new component was ejected
2 months after the 2013 epoch, visible also in our 22 GHz map presented here.
Consequently this was located upstream of the core during the brightness
temperature peak. These observations confirm that the previously detected
extreme brightness temperature in 3C 273, exceeding the inverse Compton limit,
is a short-lived phenomenon caused by a temporary departure from equipartition.
Thus, the availability of interferometric baselines capable of providing
{\mu}as angular resolution does not systematically imply measured brightness
temperatures over the known physical limits for astrophysical sources.
