The awakening of BL Lacertae: observations by Fermi, Swift, and the
  GASP-WEBT
  Since the launch of the Fermi satellite, BL Lacertae has been moderately
active at gamma-rays and optical frequencies until May 2011, when the source
started a series of strong flares. The exceptional optical sampling achieved by
the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope
(WEBT) in collaboration with the Steward Observatory allows us to perform a
detailed comparison with the daily gamma-ray observations by Fermi. Discrete
correlation analysis between the optical and gamma-ray emission reveals
correlation with a time lag of 0 +- 1 d, which suggests cospatiality of the
corresponding jet emitting regions. A better definition of the time lag is
hindered by the daily gaps in the sampling of the extremely fast flux
variations. In general, optical flares present more structure and develop on
longer time scales than corresponding gamma-ray flares. Observations at X-rays
and at millimetre wavelengths reveal a common trend, which suggests that the
region producing the mm and X-ray radiation is located downstream from the
optical and gamma-ray-emitting zone in the jet. The mean optical degree of
polarisation slightly decreases over the considered period and in general it is
higher when the flux is lower. The optical electric vector polarisation angle
(EVPA) shows a preferred orientation of about 15 deg, nearly aligned with the
radio core EVPA and mean jet direction. Oscillations around it increase during
the 2011-2012 outburst. We investigate the effects of a geometrical
interpretation of the long-term flux variability on the polarisation. A helical
magnetic field model predicts an evolution of the mean polarisation that is in
reasonable agreement with the observations. These can be fully explained by
introducing slight variations in the compression factor in a transverse shock
waves model.
