WEBT multiwavelength monitoring and XMM-Newton observations of 
BL Lacertae in 2007–2008^*

Unveiling different emission components

¹ 
INAF, Osservatorio Astronomico di Torino, Italy e-mail: [raiteri;villata]@oato.inaf.it
² Department of Astronomy, University of Michigan, MI, USA 

³ Max-Planck-Institut für Radioastronomie, Bonn, Germany 

⁴ Osservatorio Astronomico della Regione Autonoma Valle d'Aosta, Italy 

⁵ Harvard-Smithsonian Center for Astroph., Cambridge, MA, USA 

⁶ Astron. Inst., St.-Petersburg State Univ., Russia 

⁷ Pulkovo Observatory, St. Petersburg, Russia 

⁸ Michael Adrian Observatory, Trebur, Germany 

⁹ Tuorla Observatory, Dept. of Physics and Astronomy, Univ. of Turku, Piikkiö, Finland 

¹⁰ Inst. of Astronomy, Bulgarian Academy of Sciences, Sofia, Bulgaria 

¹¹ Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain 

¹² Instituto de Astronomía, Universidad Nacional Autónoma de México, Mexico 

¹³ Department of Physics and Astronomy, Ohio Univ., OH, USA 

¹⁴ INAF, Osservatorio Astrofisico di Catania, Italy 

¹⁵ SISSA-ISAS, Trieste, Italy 

¹⁶ Armenzano Astronomical Observatory, Italy 

¹⁷ Université de Bordeaux, Observatoire Aquitain des Sciences de l'Univers, Floirac, France 

¹⁸ CNRS, Laboratoire d'Astrophysique de Bordeaux – UMR 5804, Floirac, France 

¹⁹ Institute of Astronomy, National Central University, Taiwan 

²⁰ Agrupació Astronòmica de Sabadell, Spain 

²¹ ARIES, Manora Peak, Nainital, India 

²² ZAH, Landessternwarte Heidelberg, Heidelberg, Germany 

²³ Inst. de Astrofísica de Andalucía, CSIC, Spain 

²⁴ Inst. for Astrophysical Research, Boston University, MA, USA 

²⁵ Abastumani Astrophysical Observatory, Georgia 

²⁶ Metsähovi Radio Obs., Helsinki Univ. of Technology, Finland 

²⁷ INAF, Istituto di Radioastronomia, Sezione di Noto, Italy 

²⁸ Sofia University, Bulgaria 

²⁹ Korea Astronomy and Space Science Institute, South Korea 

³⁰ Circolo Astrofili Talmassons, Italy 

³¹ Institute for Astrophysical Research, Boston University, MA, USA 

³² Nordic Optical Telescope, Santa Cruz de La Palma, Spain 

³³ Dept. of Physics and Astronomy, Univ. of Victoria, Victoria, Canada 

³⁴ Dept. of Phys., Univ. of Colorado Denver, Denver, CO USA 

³⁵ Radio Astronomy Lab. of Crimean Astrophysical Observatory, Ukraine

Received: 22 July 2009
Accepted: 3 September 2009

Abstract

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 energy 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 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-ray 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 and a luminosity .