BL Lacertae is a well-known source that has been observed in the optical band for more than a century. It has been used to define a whole class of active galactic nuclei (AGNs), which are characterized by absence or extreme weakness of the emission lines, intense variability at all wavelengths, high polarization, and superluminal motion of radio components. The BL Lacertae objects, together with the flat-spectrum radio quasars, are known as "blazars''.
Although the details of blazar emission are under debate, the commonly accepted
general scenario foresees a central black hole fed by an accretion disc,
and a plasma jet which is responsible for the non-thermal continuum.
In order to explain several observational pieces of evidence, the emitted radiation
is assumed to be relativistically beamed towards us.
The low-energy emission, from the
radio band to the UV-X-ray region, is likely synchrotron radiation produced by
ultra-relativistic electrons in the jet. The origin of the higher-energy
radiation, up to 
-rays, is less clearly established: it is
reasonable to suppose that the soft radiation produced by the synchrotron
process can be inversely comptonized up to the -ray energies (SSC
models), but it is also possible that the photons to be comptonized come from out of
the jet, either directly from the accretion disc or from the broad line region
(EC models). Recent observations seem to indicate that a mixture of SSC and EC
processes may be at work in the blazar jets (see e.g. Madejski et al. 1999 and Böttcher & Bloom 2000 for the case of BL Lacertae).
Another possibility is that the high-energy emission is produced by pair
cascades coming from the interaction between soft photons and highly
relativistic protons (proton models).
The violent flux variations observed in blazars have been explained in a variety of ways: shocks travelling down the jet (e.g. Marscher 1996), changes of the Doppler factor due to geometrical reasons (e.g. Dreissigacker & Camenzind 1996; Villata & Raiteri 1999), accretion disc instabilities (e.g. Wiita 1996), gravitational microlensing (e.g. Schneider & Weiss 1987). The observation of microvariability, that is of flux changes on time scales of less than a day, raises the question of what is the smallest time scale of variability in blazars and, if the variations are of intrinsic nature, of how small the size of the emitting region can be.
Variability studies are thus a powerful tool to investigate blazar emission and to discriminate among the various theoretical interpretations, in particular when observations are done in a continuous way and simultaneously at different wavelengths. This is why in the last years optical observers have set up collaborations to make the observational effort more efficient.
The Whole Earth Blazar Telescope (WEBT;
http://www.to.astro.it/blazars/webt/) is an international
organization that includes about 30 observatories located all around the
world. Its aim is to obtain accurate and continuous monitoring of a source
during a time-limited campaign (from few days to several weeks), which is
often organized in concert with satellite observations in the X- and
-rays, and ground-based observations in the radio band and at TeV
energies. The location at different longitudes of its members allows them to
optimize observations during the 24 hours of the day, gaps due to daylight being, in
theory, extremely small. In practice, limitations due to bad weather conditions,
telescope overscheduling, technical problems are present, but in any case this
monitoring strategy has already demonstrated that it can provide unprecedentedly dense
sampling (see Villata et al. 2000 about the WEBT campaign on S5
0716+71 of February 1999, and Raiteri et al. 2001 about the
first-light WEBT campaign on AO 0235+16 of November 1997), and even better
results are expected by the robotization of at least some of the telescopes
participating in the WEBT.
In this paper we report on UBVRI photometric monitoring of BL Lacertae during the summer 2000 WEBT campaign and its extension (May 2000-January 2001). The core optical campaign took place simultaneously with the planned high-energy campaign coordinated by M. Böttcher, involving X-ray and TeV observatories such as BeppoSAX, RXTE, CAT, and HEGRA (Böttcher et al. 2002). Previous WEBT campaigns on BL Lacertae had been organized in June 1999, in conjunction with observations by the BeppoSAX and ASCA satellites (Mattox 1999). The results of these campaigns are presented in Ravasio et al. (2002) and Villata et al. (2002).
The present paper is organized as follows: in Sect. 2 we review optical studies on BL Lacertae; the observing strategy and data reduction/assembling procedures are described in Sect. 3. UBVRI light curves are presented in Sect. 4, colour indexes are discussed in Sect. 5, and the autocorrelation study can be found in Sect. 6. Discussion and conclusions are drawn in Sect. 7.
Copyright ESO 2002