Multiwavelength behaviour of the blazar 3C279: decade-long study from
  $\gamma$-ray to radio
  We report the results of decade-long (2008-2018) $\gamma$-ray to 1 GHz radio
monitoring of the blazar 3C 279, including GASP/WEBT, $\it{Fermi}$ and
$\it{Swift}$ data, as well as polarimetric and spectroscopic data. The X-ray
and $\gamma$-ray light curves correlate well, with no delay > 3 hours, implying
general co-spatiality of the emission regions. The $\gamma$-ray-optical
flux-flux relation changes with activity state, ranging from a linear to a more
complex dependence. The behaviour of the Stokes parameters at optical and radio
wavelengths, including 43 GHz VLBA images, supports either a predominantly
helical magnetic field or motion of the radiating plasma along a spiral path.
Apparent speeds of emission knots range from 10 to 37c, with the highest values
requiring bulk Lorentz factors close to those needed to explain $\gamma$-ray
variability on very short time scales. The Mg II emission line flux in the
`blue' and `red' wings correlates with the optical synchrotron continuum flux
density, possibly providing a variable source of seed photons for inverse
Compton scattering. In the radio bands we find progressive delays of the most
prominent light curve maxima with decreasing frequency, as expected from the
frequency dependence of the $\tau=1$ surface of synchrotron self-absorption.
The global maximum in the 86 GHz light curve becomes less prominent at lower
frequencies, while a local maximum, appearing in 2014, strengthens toward
decreasing frequencies, becoming pronounced at $\sim5$ GHz. These tendencies
suggest different Doppler boosting of stratified radio-emitting zones in the
jet.
