Astronomy & Astrophysics

Free access article

A&A 424, 497-507 (2004)
DOI: 10.1051/0004-6361:20040439

The WEBT campaigns on BL Lacertae

Time and cross-correlation analysis of optical and radio light curves 1968-2003

M. Villata¹, C. M. Raiteri¹, H. D. Aller², M. F. Aller², H. Teräsranta³, P. Koivula³, S. Wiren³, O. M. Kurtanidze^{4, 5, 6}, M. G. Nikolashvili⁴, M. A. Ibrahimov⁷, I. E. Papadakis^{8, 9}, G. Tosti¹⁰, F. Hroch¹¹, L. O. Takalo¹², A. Sillanpää¹², V. A. Hagen-Thorn^{13, 14}, V. M. Larionov^{13, 14}, R. D. Schwartz¹⁵, J. Basler¹⁵, L. F. Brown¹⁶ and T. J. Balonek¹⁷

¹ Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Torino, via Osservatorio 20, 10025 Pino Torinese (TO), Italy
e-mail: villata@to.astro.it
² Department of Astronomy, Dennison Building, University of Michigan, Ann Arbor, MI 48109, USA
³ Metsähovi Radio Observatory, Helsinki University of Technology, Metsähovintie 114, 02540 Kylmälä, Finland
⁴ Abastumani Astrophysical Observatory, 383762 Abastumani, Georgia
⁵ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁶ Landessternwarte Heidelberg-Königstuhl, Königstuhl 12, 69117 Heidelberg, Germany
⁷ Ulugh Beg Astronomical Institute, Academy of Sciences of Uzbekistan, 33 Astronomical Str., Tashkent 700052, Uzbekistan
⁸ IESL, FORTH, 711 10 Heraklion, Crete, Greece
⁹ Physics Department, University of Crete, PO Box 2208, 710 03 Heraklion, Crete, Greece
¹⁰ Dipartimento di Fisica, Università di Perugia, via A. Pascoli, 06123 Perugia, Italy
¹¹ Institute of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlárská 2, 611 37 Brno, Czech Republic
¹² Tuorla Observatory, 21500 Piikkiö, Finland
¹³ Astronomical Institute, St.-Petersburg State University, Universitetsky Pr. 28, Petrodvoretz, 198504 St.-Petersburg, Russia
¹⁴ Isaac Newton Institute of Chile, St.-Petersburg Branch
¹⁵ Department of Physics and Astronomy, University of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, MO 63121, USA
¹⁶ Department of Physics, Astronomy and Geophysics, Connecticut College, New London, CT 06320, USA
¹⁷ Foggy Bottom Observatory, Colgate University, 13 Oak Drive, Hamilton, NY 13346, USA

(Received 15 March 2004 / Accepted 27 May 2004 )

Abstract
The Whole Earth Blazar Telescope (WEBT) collaboration has collected a large amount of optical and radio data on BL Lacertae in the last years, which, when added to literature data, allow to construct well-sampled light curves of the source from 1968 to the end of 2003. These optical and radio data are here analysed with three statistical methods designed for unevenly-sampled data trains in order to search for possible periodicities. While the main radio outbursts repeat every ~ 8 years, with a possible progressive stretching of the period, the evidence of an optical periodicity is much less clear. Radio light curves from 4.8 to 37 GHz are well correlated, with variations at the higher frequencies leading the lower-frequency ones by a few weeks for contiguous bands, up to a few months when considering the largest frequency separations. The radio behaviour reveals the presence of two different components, the softer-spectrum one constituting the bulk of the radio emission. On the other hand, the harder component shows itself as radio events which appear enhanced at the higher frequencies and seem to have optical counterparts. Cross-correlation between the optical light curve and radio hardness ratios indicates a radio time delay of more than 3 months. Thus, our analysis suggests a scenario where flux variations propagate towards less and less opaque regions, giving rise to related optical and hard radio events and, in more extended zones, to soft events apparently uncorrelated with the former ones.

Key words: galaxies: BL Lacertae objects: general -- galaxies: BL Lacertae objects: individual: BL Lacertae -- galaxies: jets -- galaxies: quasars: general

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