Exceptional outburst of the blazar CTA 102 in 2012: The GASP-WEBT
  campaign and its extension
  After several years of quiescence, the blazar CTA 102 underwent an
exceptional outburst in 2012 September-October. The flare was tracked from
gamma-ray to near-infrared frequencies, including Fermi and Swift data as well
as photometric and polarimetric data from several observatories. An intensive
GASP-WEBT collaboration campaign in optical and NIR bands, with an addition of
previously unpublished archival data and extension through fall 2015, allows
comparison of this outburst with the previous activity period of this blazar in
2004-2005. We find remarkable similarity between the optical and gamma-ray
behaviour of CTA 102 during the outburst, with a time lag between the two light
curves of ~1 hour, indicative of co-spatiality of the optical and gamma-ray
emission regions. The relation between the gamma-ray and optical fluxes is
consistent with the SSC mechanism, with a quadratic dependence of the SSC
gamma-ray flux on the synchrotron optical flux evident in the post-outburst
stage. However, the gamma-ray/optical relationship is linear during the
outburst; we attribute this to changes in the Doppler factor. A strong
harder-when-brighter spectral dependence is seen both the in gamma-ray and
optical non-thermal emission. This hardening can be explained by convexity of
the UV-NIR spectrum that moves to higher frequencies owing to an increased
Doppler shift as the viewing angle decreases during the outburst stage. The
overall pattern of Stokes parameter variations agrees with a model of a
radiating blob or shock wave that moves along a helical path down the jet.
