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In 2007-2008 we carried out a new multiwavelength campaign of the Whole EarthBlazar Telescope (WEBT) on BL Lacertae, involving three pointings by theXMM-Newton satellite, to study its emission properties. The source wasmonitored in the optical-to-radio bands by 37 telescopes. The brightness levelwas relatively low. Some episodes of very fast variability were detected in theoptical bands. The X-ray spectra are well fitted by a power law with photonindex of about 2 and photoelectric absorption exceeding the Galactic value.However, when taking into account the presence of a molecular cloud on the lineof sight, the data are best fitted by a double power law, implying a concaveX-ray spectrum. The spectral energy distributions (SEDs) built withsimultaneous radio-to-X-ray data at the epochs of the XMM-Newton observationssuggest that the peak of the synchrotron emission lies in the near-IR band, andshow a prominent UV excess, besides a slight soft-X-ray excess. A comparisonwith the SEDs corresponding to previous observations with X-ray satellitesshows that the X-ray spectrum is extremely variable. We ascribe the UV excessto thermal emission from the accretion disc, and the other broad-band spectralfeatures to the presence of two synchrotron components, with their related SSCemission. We fit the thermal emission with a black body law and the non-thermalcomponents by means of a helical jet model. The fit indicates a disctemperature greater than 20000 K and a luminosity greater than 6 x 10^44 erg/s.

WEBT multiwavelength monitoring and XMM-Newton observations of BL Lacertae in 2007-2008. Unveiling different emission components