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

• Published in: Astronomy and astrophysics (Berlin) 2009-11, Vol.507 (2), p.769-779
• Main Authors: RAITERI, C. M, VILLATA, M, STRIGACHEV, A, AGUDO, I, ALLER, H. D, BACHEV, R, BENITEZ, E, BERDYUGIN, A, BÖTTCHER, M, BUEMI, C. S, BUTTIGLIONE, S, CAROSATI, D, CAPETTI, A, CHARLOT, P, CHEN, W. P, DULTZIN, D, FORNE, E, FUHRMANN, L, GOMEZ, J. L, GUPTA, A. C, HEIDT, J, HIRIART, D, HSIAO, W.-S, ALLER, M. F, JELINEK, M, JORSTAD, S. G, KIMERIDZE, G. N, KONSTANTINOVA, T. S, KOPATSKAYA, E. N, KOSTOV, A, KURTANIDZE, O. M, LÄHTEENMÄKI, A, LANTERI, L, LARIONOVA, L. V, BACH, U, LETO, P, LATEV, G, LE CAMPION, J.-F, LEE, C.-U, LIGUSTRI, R, LINDFORS, E, MARSCHER, A. P, MIHOV, B, NIKOLASHVILI, M. G, NIKOLOV, Y, CALCIDESE, P, OVCHAROV, E, PRINCIPE, D, PURSIMO, T, RAGOZZINE, B, ROBB, R. M, ROS, J. A, SADUN, A. C, SAGAR, R, SEMKOV, E, SIGUA, L. A, GURWELL, M. A, LARIONOV, V. M, OHLERT, J, NILSSON, K
• Format: Article
• Language: English
• Subjects: Astrophysics; High Energy Astrophysical Phenomena; Physics; Sciences of the Universe
• ISSN: 0004-6361; 1432-0746; 1432-0756
• DOI: 10.1051/0004-6361/200912953

In 2007-2008 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, to study its emission properties. 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. The X-ray spectra are well fitted by a power law with photon index of about 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 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 extremely variable. 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 greater than 20000 K and a luminosity greater than 6 x 10^44 erg/s.