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We perform a multi-wavelength polarimetric study of the quasar CTA 102 duringan extraordinarily bright $\gamma$-ray outburst detected by the {\it Fermi}Large Area Telescope in September-October 2012 when the source reached a fluxof F$_{>100~\mathrm{MeV}} =5.2\pm0.4\times10^{-6}$ photons cm$^{-2}$ s$^{-1}$.At the same time the source displayed an unprecedented optical and NIRoutburst. We study the evolution of the parsec scale jet with ultra-highangular resolution through a sequence of 80 total and polarized intensity VeryLong Baseline Array images at 43 GHz, covering the observing period from June2007 to June 2014. We find that the $\gamma$-ray outburst is coincident withflares at all the other frequencies and is related to the passage of a newsuperluminal knot through the radio core. The powerful $\gamma$-ray emission isassociated with a change in direction of the jet, which became oriented moreclosely to our line of sight ($\theta\sim$1.2$^{\circ}$) during the ejection ofthe knot and the $\gamma$-ray outburst. During the flare, the optical polarizedemission displays intra-day variability and a clear clockwise rotation ofEVPAs, which we associate with the path followed by the knot as it moves alonghelical magnetic field lines, although a random walk of the EVPA caused by aturbulent magnetic field cannot be ruled out. We locate the $\gamma$-rayoutburst a short distance downstream of the radio core, parsecs from the blackhole. This suggests that synchrotron self-Compton scattering of near-infraredto ultraviolet photons is the probable mechanism for the $\gamma$-rayproduction.

A multi-wavelength polarimetric study of the blazar CTA 102 during a Gamma-ray flare in 2012