15 625 (=56) observations were performed in the period May 2000-January 2001 by 24 telescopes of the WEBT collaboration to monitor the optical variability of BL Lacertae.
An exceptional sampling was reached, especially during the core WEBT campaign (July 17-August 11, 2000), where time gaps were limited to a few hours and they were essentially due to the lack of observers in the Pacific area.
Well-defined intranight trends were detected, with variations up to
in
less than 
.
Colour index analysis performed after subtracting the host galaxy contribution from fluxes suggests the existence of at least two variability mechanisms: the first one is essentially achromatic, and is responsible for the long-term trend, while the second one, causing the fast flares superposed on the long-term trend, implies spectral changes, the spectrum becoming flatter when the source gets brighter. A similar behaviour was already found in the BL Lac object S5 0716+71 by Ghisellini et al. (1997).
As mentioned in the Introduction, Clements & Carini (2001) argued that the bluer-when-brighter behaviour might be due to a greater host galaxy contribution when the AGN is fainter. Since we subtracted the host galaxy contribution and still found the bluer-when-brighter trend (and only for short-term variations), we conclude that spectral changes are not related to the host galaxy contribution, but are an intrinsic property of fast flares.
The behaviour of the achromatic component determining the flux base-level
modulations can be interpreted in terms of variation of the relativistic Doppler
factor 
.
By assuming that fluxes are enhanced proportionally to
by relativistic boosting, a maximum variation of
of a
factor 1.36 is inferred. This change in
can be due to
either a viewing angle variation of few degrees (
-
)
or a noticeable change of the bulk Lorentz factor (more than
50%). From this point of view our interpretation would suggest that the flux
base-level modulations are more likely explained by a geometrical effect than
by an energetic one.
In a geometrical scenario such as that proposed by Villata & Raiteri
(1999), where viewing angle variation is due to rotation of an
inhomogeneous helical jet, a rate of few
degrees (say, 1-
)
in a month (as can be inferred from the steepest
base-level variations, see Fig. 9) would imply an upper limit to
a possible periodicity of 10-
.
Indeed, in that model, a perfect
helix would give rise to a perfect periodicity of well-defined outbursts; if
the helical path presents distortions, the outburst phase (which can last for a
significant fraction of the period) is consequently disturbed by modulated
events whose steepest variations represent a lower limit to the rotation
rate.
We performed discrete autocorrelation analysis in
order to search for the existence of intermediate-short characteristic time
scales in our observing period. Variability on a typical time
scale of 
was found
during the core WEBT campaign.
Cross-correlation analysis on the B and R fluxes does not reveal any significant time lag between variations in the two bands.
Acknowledgements
This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This work was partly supported by the Italian Ministry for University and Research (MURST) under grant Cofin 2001/028773 and by the Italian Space Agency (ASI) under contract CNR-ASI 1/R/27/00. It is based partly on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway and Sweden in the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofísica de Canarias. St.-Petersburg group was supported by the Federal Program "Integration'' under grants K0232 and A0007. J. Basler acknowledges support by the NASA Missouri Space Grant Consortium. J. H. Fan's work is partially supported by NSFC (19973001).
Copyright ESO 2002