  X-ray data from the blazar BL Lac are used to investigate the nature of its
variability, and more precisely the flux dependency of the variability and the
distribution of fluxes. The variations in the flux are found to have a
lognormal distribution and the average amplitude of variability is proportional
to the flux level. BL Lac is the first blazar in which lognormal X-ray
variability is clearly detected. Lognormal variability in X-ray light curves,
probably related to accretion disk activity, has been discovered in various
compact systems, such as Seyfert galaxies and X-ray binaries. The light curve
is orders of magnitude less variable than other blazars, with few bursting
episodes. If this defines a specific state of the source, then the lognormality
might be the imprint of the accretion disk on the jet, linking for the first
time accretion and jet properties in a blazar.

Cosmology and Nongalactic Astrophysics;High Energy Astrophysical Phenomena