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Since the launch of the Fermi satellite, BL Lacertae has been moderatelyactive at gamma-rays and optical frequencies until May 2011, when the sourcestarted a series of strong flares. The exceptional optical sampling achieved bythe GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope(WEBT) in collaboration with the Steward Observatory allows us to perform adetailed comparison with the daily gamma-ray observations by Fermi. Discretecorrelation analysis between the optical and gamma-ray emission revealscorrelation with a time lag of 0 +- 1 d, which suggests cospatiality of thecorresponding jet emitting regions. A better definition of the time lag ishindered by the daily gaps in the sampling of the extremely fast fluxvariations. In general, optical flares present more structure and develop onlonger time scales than corresponding gamma-ray flares. Observations at X-raysand at millimetre wavelengths reveal a common trend, which suggests that theregion producing the mm and X-ray radiation is located downstream from theoptical and gamma-ray-emitting zone in the jet. The mean optical degree ofpolarisation slightly decreases over the considered period and in general it ishigher when the flux is lower. The optical electric vector polarisation angle(EVPA) shows a preferred orientation of about 15 deg, nearly aligned with theradio core EVPA and mean jet direction. Oscillations around it increase duringthe 2011-2012 outburst. We investigate the effects of a geometricalinterpretation of the long-term flux variability on the polarisation. A helicalmagnetic field model predicts an evolution of the mean polarisation that is inreasonable agreement with the observations. These can be fully explained byintroducing slight variations in the compression factor in a transverse shockwaves model.

The awakening of BL Lacertae: observations by Fermi, Swift, and the GASP-WEBT