The optical-gamma correlation in BL Lacertae

We present multifrequency light curves of BL Lacertae from February 2008 to October 2012. Low-energy data (optical and millimetre) were acquired in the framework of a GASP-WEBT project. High-energy data (ultraviolet, X-ray, and gamma-ray) come from observations of the Swift, RXTE, and Fermi satellites. After a period of moderate activity, in May 2011 the source suddenly started to flare at gamma and optical-UV frequencies. Activity at millimetre wavelengths and X rays began 3-4 months later. This behaviour offered a good opportunity to study the correlation among flux variability in different bands, in particular between the best-sampled optical and gamma-ray light curves. However, even in this fortuitous case, we can only define a general correlation with likely no time lag, but with a lag uncertainty of +/- 1 day. Indeed, the data reveal a complex relationship between the gamma and optical fluxes, which cannot be unveiled because of the small gaps in the sampling of this extremely variable source.