Volume 507, Number 2, November IV 2009
|Page(s)||769 - 779|
|Published online||15 September 2009|
WEBT multiwavelength monitoring and XMM-Newton observations of BL Lacertae in 2007–2008*
Unveiling different emission components
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
Accepted: 3 September 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 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 .
Key words: galaxies: active / galaxies: BL Lacertae objects: general / galaxies: BL Lacertae objects: individual: BL Lacertae / galaxies: jets
© ESO, 2009
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