The WEBT campaign to observe AO 0235+16 in the 2003-2004 observing season

A multiwavelength campaign on the BL Lac object AO 0235+16 has been organized by the WEBT collaboration during the observing seasons 2003-2004 and 2004-2005. Here we report on the results of the first season, which saw the participation of 24 optical and near-IR telescopes and 4 radio telescopes, as well as the first XMM-Newton pointing, which occurred on January 18-19, 2004. Unpublished data from previous epochs were also collected. In the optical and near-IR the source has been very active in the last 3 years, though rather faint most of the time, with noticeable variations of more than a mag in a few days. In contrast, in the radio bands it appears "quiescent" since early 2000. The X-ray spectra obtained by the three EPIC detectors are well fitted by a power law with extra-absorption at z=0.524. No significant variation of the X-ray flux occurred during the pointing. In contrast, contemporaneous dense radio monitoring with the 100 m telescope at Effelsberg shows a ~2-3% flux decrease in 6-7 hours, which, if intrinsic, would imply a brightness temperature well above the Compton limit. We construct the broad-band spectral energy distribution of January 18-19, 2004 with simultaneous radio data from Effelsberg, optical data from the Nordic Optical Telescope, optical-UV data from the Optical Monitor onboard XMM-Newton, and X-ray data by the EPIC instruments. Particular care was taken to correct data for extinction due to both the Milky Way and the z=0.524 absorber. The resulting SED suggests the existence of a bump in the UV spectral region.