Blazar spectral variability as explained by a twisting inhomogeneous jet
  Blazar emission is dominated by non-thermal radiation from a relativistic jet
pointing toward us, therefore undergoing Doppler beaming. This is responsible
for flux enhancement and contraction of the variability time scales, so that
most blazars appear as luminous sources characterized by noticeable and fast
flux changes at all frequencies. The mechanisms producing their unpredictable
variability are debated and include injection, acceleration and cooling of
particles, with possible intervention of shock waves or turbulence. Changes in
the viewing angle of the emitting knots or jet regions have also been suggested
to explain flaring events or specific properties such as intraday variability,
quasi-periodicities, or the delay of radio flux variations relative to optical
changes. However, such a geometric interpretation has not been universally
accepted because alternative explanations based on changes of physical
conditions can also work in many cases. Here we report the results of
optical-to-radio monitoring of the blazar CTA 102 by the Whole Earth Blazar
Telescope Collaboration and show that the observed long-term flux and spectral
variability is best explained by an inhomogeneous, curved jet that undergoes
orientation changes. We propose that magnetohydrodynamic instabilities or
rotation of a twisted jet cause different jet regions to change their
orientation and hence their relative Doppler factors. In particular, the recent
extreme optical outburst (six magnitudes) occurred when the corresponding jet
emitting region acquired a minimum viewing angle.
