Relativistic Outflow Drives Gamma-Ray Emission in 3C345
  Aims: 3C345 was recently identified as a gamma-ray emitter, based on the
first 20 months of Fermi-LAT data and optical monitoring. In this paper, a
connection between the gamma-ray and optical variability of 3C345 and
properties of its parsec-scale radio emission is investigated. Methods: The
Fermi-LAT data of 3C345, covering an energy range of 0.1-300 GeV, were combined
with 32 Very Long Baseline Array observations of the object made at 43.2 GHz in
the period of January 2008 - March 2010. Results: The VLBA data reveal
morphology and kinematics of the flow on scales of up to ~5 milliarcseconds
(mas; deprojected linear distances of 380 parsecs). The brightness temperature,
T_b(r), measured along the jet first decreases with distance proportional to
r^-(0.95 +/-0.69) and later exhibits a break at ~0.3 mas, with T_b(r)
proportional to r^-(4.11 +/-0.85) at larger separations. Variations of the
gamma-ray, optical and parsec-scale radio emission show a similar long-term
trend persistent during the entire VLBA monitoring period. The gamma-ray and
optical variations on shorter time scales are related to structural changes in
the jet on scales of ~0.3 mas (~23 parsecs, deprojected), with the gamma-ray
and optical flares possibly related to the evolution of four distinct
superluminal components identified in the flow. Conclusions: The observations
indicate that both the quiescent and flaring components of the gamma-ray
emission are produced in a region of the jet of ~23 pc in extent. This region
may mark the Compton-loss dominated zone of the flow and its large extent may
favor the synchrotron self-Compton mechanism for gamma-ray production in the
relativistic jet of the quasar 3C345.
