  CTA 102, classified as a flat spectrum radio quasar at z=1.037, produced
exceptionally bright optical flare in 2012 September. Following Fermi-LAT
detection of enhanced gamma-ray activity, we densely monitored this source in
the optical and near-infrared bands for the subsequent ten nights using twelve
telescopes in Japan and South-Africa. On MJD 56197 (2012 September 27, 4-5 days
after the peak of bright gamma-ray flare), polarized flux showed a transient
increase, while total flux and polarization angle remained almost constant
during the "orphan polarized-flux flare". We also detected an intra-night and
prominent flare on MJD 56202. The total and polarized fluxes showed quite
similar temporal variations, but PA again remained constant during the flare.
Interestingly, the polarization angles during the two flares were significantly
different from the jet direction. Emergence of a new emission component with
high polarization degree (PD) up to 40% would be responsible for the observed
two flares, and such a high PD indicates a presence of highly ordered magnetic
field at the emission site. We discuss that the well-ordered magnetic field and
even the observed directions of polarization angle which is grossly
perpendicular to the jet are reasonably accounted for by transverse shock(s)
propagating down the jet.
