PKS 1502+106: A high-redshift Fermi blazar at extreme angular
  resolution. Structural dynamics with VLBI imaging up to 86 GHz
  Context. Blazars are among the most energetic objects in the Universe. In
2008 August, Fermi/LAT detected the blazar PKS 1502+106 showing a rapid and
strong gamma-ray outburst followed by high and variable flux over the next
months. This activity at high energies triggered an intensive multi-wavelength
campaign covering also the radio, optical, UV, and X-ray bands indicating that
the flare was accompanied by a simultaneous outburst at optical/UV/X-rays and a
delayed outburst at radio bands.
  Aims: In the current work we explore the phenomenology and physical
conditions within the ultra-relativistic jet of the gamma-ray blazar PKS
1502+106. Additionally, we address the question of the spatial localization of
the MeV/GeV-emitting region of the source.
  Methods: We utilize ultra-high angular resolution mm-VLBI observations at 43
and 86 GHz complemented by VLBI observations at 15 GHz. We also employ
single-dish radio data from the F-GAMMA program at frequencies matching the
VLBI monitoring.
  Results: PKS 1502+106 shows a compact core-jet morphology and fast
superluminal motion with apparent speeds in the range 5--22 c. Estimation of
Doppler factors along the jet yield values between ~7 up to ~50. This Doppler
factor gradient implies an accelerating jet. The viewing angle towards the
source differs between the inner and outer jet, with the former at ~3 degrees
and the latter at ~1 degree, after the jet bends towards the observer beyond 1
mas. The de-projected opening angle of the ultra-fast, magnetically-dominated
jet is found to be (3.8 +/- 0.5) degrees. A single jet component can be
associated with the pronounced flare both at high-energies and in radio bands.
Finally, the gamma-ray emission region is localized at less than 5.9 pc away
from the jet base.
