C. M. Raiteri, et al. (incl. M. Böttcher,
D. Principe, B. Ragozzine, and K. Uckert),
Astronomy & Astrophysics, in press (2009)
Context.BL Lacertae is the prototype of the blazar subclass named
after it. Yet, it has occasionally shown a peculiar behaviour that has
questioned a simple interpretation of its broad-band emission in terms
of synchrotron plus synchrotron self-Compton (SSC) radiation.
Aims.In the 2007 - 2008 observing season we carried out a new
multiwavelength campaign of the Whole Earth Blazar Telescope (WEBT) on
BL Lacertae, involving three pointings by the XMM-Newton satellite in July
and December 2007, and January 2008, to study its emission properties,
particularly in the optical - X-ray range.
Methods.The source was monitored in the optical to radio bands
by 37 telescopes. The brightness level was relatively low. Some episodes
of very fast variability were detected in the optical bands. Flux changes
had larger amplitude at the higher radio frequencies than at longer
wavelengths.
Results.The X-ray spectra acquired by the EPIC instrument onboard
XMM-Newton are well fitted by a power-law with photon index Gamma ~ 2
and photoelectric absorption exceeding the Galactic value. However, when
taking into account the presence of a molecular cloud on the line of
sight, the EPIC data are best fitted by a double power-law, implying a
concave X-ray spectrum. The spectral energy distributions (SEDs) built
with simultaneous radio to X-rya data at the epochs of the XMM-Newton
observations suggest that the peak of the synchrotron emission lies in
the near-IR band, and show a prominent UV excess, besides a slight soft
X-ray excess. A comparison with the SEDs corresponding to previous observations
with X-ray satellites shows that the X-ray spectrum is very variable,
since it can change from extremely steep to extremely hard, and
can be more or less curved in intermediate states. We ascribe the UV
excess to thermal emission from the accretion disc, and the other
broad-band spectral features to the presence of two synchrotron components,
with their related SSC emission. We fit the thermal emission with a
black body law and the non-thermal components by means of a helical
jet model. The fit indicates a disc temperature above 20,000 K and a
luminosity above 6X1044 erg/s.