3C273 variability at 7 mm: Evidences of shocks and precession in the jet
  We report 4 years of observations of 3C273 at 7 mm obtained with the
Itapetinga Radiotelescope, in Brazil, between 2009 and 2013. We detected a
flare in 2010 March, when the flux density increased by 50% and reached 35 Jy.
After the flare, the flux density started to decrease and reached values lower
than 10 Jy. We suggest that the 7 mm flare is the radio counterpart of the
$\gamma$-ray flare observed by Fermi/LAT in 2009 September, in which the flux
density at high energies reached a factor of fifty of its average value. A
delay of 170 days between the radio and $\gamma$-ray flares was revealed using
the Discrete Correlation Function (DCF) that can be interpreted in the context
of a shock model, in which each flare corresponds to the formation of a compact
superluminal component that expands and becomes optically thin at radio
frequencies at latter epochs. The difference in flare intensity between
frequencies and at a different times, is explained as a consequence of an
increase in the Doppler factor $\delta$, as predicted by the 16 year precession
model proposed by Abraham & Romero, which has a large effect on boosting at
high frequencies while does not affect too much the observed optically thick
radio emission. We discuss other observable effects of the variation in
$\delta$, as the increase in the formation rate of superluminal components, the
variations in the time delay between flares and the periodic behaviour of the
radio light curve that we found compatible with changes in the Doppler factor.
