Variability of the blazar 4C 38.41 (B3 1633+382) from GHz frequencies to
  GeV energies
  The quasar-type blazar 4C 38.41 (B3 1633+382) experienced a large outburst in
2011, which was detected throughout the entire electromagnetic spectrum. We
present the results of low-energy multifrequency monitoring by the GASP project
of the WEBT consortium and collaborators, as well as those of
spectropolarimetric/spectrophotometric monitoring at the Steward Observatory.
We also analyse high-energy observations of the Swift and Fermi satellites. In
the optical-UV band, several results indicate that there is a contribution from
a QSO-like emission component, in addition to both variable and polarised jet
emission. The unpolarised emission component is likely thermal radiation from
the accretion disc that dilutes the jet polarisation. We estimate its
brightness to be R(QSO) ~ 17.85 - 18 and derive the intrinsic jet polarisation
degree. We find no clear correlation between the optical and radio light
curves, while the correlation between the optical and \gamma-ray flux
apparently fades in time, likely because of an increasing optical to \gamma-ray
flux ratio. As suggested for other blazars, the long-term variability of 4C
38.41 can be interpreted in terms of an inhomogeneous bent jet, where different
emitting regions can change their alignment with respect to the line of sight,
leading to variations in the Doppler factor \delta. Under the hypothesis that
in the period 2008-2011 all the \gamma-ray and optical variability on a
one-week timescale were due to changes in \delta, this would range between ~ 7
and ~ 21. If the variability were caused by changes in the viewing angle
\theta\ only, then \theta\ would go from ~ 2.6 degr to ~ 5 degr. Variations in
the viewing angle would also account for the dependence of the polarisation
degree on the source brightness in the framework of a shock-in-jet model.
