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A&A 421, 103-114 (2004)
DOI: 10.1051/0004-6361:20035895

The WEBT BL Lacertae Campaign 2001 and its extension

Optical light curves and colour analysis 1994-2002

M. Villata¹, C. M. Raiteri¹, O. M. Kurtanidze^{2, 3, 4}, M. G. Nikolashvili², M. A. Ibrahimov⁵, I. E. Papadakis^{6, 7}, G. Tosti⁸, F. Hroch⁹, L. O. Takalo¹⁰, A. Sillanpää¹⁰, V. A. Hagen-Thorn^{11, 12}, V. M. Larionov^{11, 12}, R. D. Schwartz¹³, J. Basler¹³, L. F. Brown¹⁴, T. J. Balonek¹⁵, E. Benítez¹⁶, A. Ramírez¹⁶, A. C. Sadun¹⁷, P. Boltwood¹⁸, M. T. Carini¹⁹, D. Barnaby¹⁹, J. M. Coloma²⁰, J. A. Ros²⁰, B. Z. Dai^{21, 22, 23}, G. Z. Xie^{21, 22, 23}, J. R. Mattox²⁴, D. Rodriguez²⁵, I. M. Asfandiyarov⁵, A. Atkerson¹⁹, J. L. Beem¹⁴, S. D. Bloom²⁶, S. M. Chanturiya², S. Ciprini⁸, S. Crapanzano¹, J. A. de Diego¹⁶, N. V. Efimova¹¹, D. Gardiol^{1, 27}, J. C. Guerra²⁷, B. B. Kahharov⁵, B. Z. Kapanadze², H. Karttunen¹⁰, T. Kato²⁸, G. N. Kimeridze², N. A. Kudryavtseva¹¹, M. Lainela¹⁰, L. Lanteri¹, E. G. Larionova¹¹, M. Maesano²⁹, N. Marchili⁸, G. Massone¹, T. Monroe¹⁹, F. Montagni³⁰, R. Nesci³¹, K. Nilsson¹⁰, J. C. Noble³², G. Nucciarelli⁸, L. Ostorero^{4, 33}, J. Papamastorakis^{7, 6}, M. Pasanen¹⁰, C. S. Peters¹⁴, T. Pursimo³⁴, P. Reig^{35, 6}, W. Ryle¹⁹, S. Sclavi³¹, L. A. Sigua², M. Uemura²⁸ and W. Wills<sup>19

1</sup>  Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Torino, via Osservatorio 20, 10025 Pino Torinese (TO), Italy
²  Abastumani Astrophysical Observatory, 383762 Abastumani, Georgia
³  Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁴  Landessternwarte Heidelberg-Königstuhl, Königstuhl 12, 69117 Heidelberg, Germany
⁵  Ulugh Beg Astronomical Institute, Academy of Sciences of Uzbekistan, 33 Astronomical Str., Tashkent 700052, Uzbekistan
⁶  IESL, FORTH, 711 10 Heraklion, Crete, Greece
⁷  Physics Department, University of Crete, PO Box 2208, 710 03 Heraklion, Crete, Greece
⁸  Dipartimento di Fisica, Università di Perugia, Via A. Pascoli, 06123 Perugia, Italy
⁹  Institute of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlárská 2, 611 37 Brno, Czech Republic
¹⁰  Tuorla Observatory, 21500 Piikkiö, Finland
¹¹  Astronomical Institute, St.-Petersburg State University, Universitetsky Pr. 28, Petrodvoretz, 198504 St.-Petersburg, Russia
¹²  Isaac Newton Institute of Chile, St.-Petersburg Branch, Russia
¹³  Department of Physics and Astronomy, University of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, MO 63121, USA
¹⁴  Department of Physics, Astronomy and Geophysics, Connecticut College, New London, CT 06320, USA
¹⁵  Foggy Bottom Observatory, Colgate University, 13 Oak Drive, Hamilton, NY 13346, USA
¹⁶  Instituto de Astronomía, UNAM, Apdo. Postal 70-264, 04510 México DF, Mexico
¹⁷  Department of Physics, University of Colorado at Denver, PO Box 173364, Denver, CO 80217-3364, USA
¹⁸  Boltwood Observatory, 1655 Main Street, Stittsville, Ontario K2S 1N6, Canada
¹⁹  Department of Physics and Astronomy, Western Kentucky University, 1 Big Red Way, Bowling Green, KY 42104, USA
²⁰  Agrupació Astronòmica de Sabadell, PO Box 50, 08200 Sabadell, Spain
²¹  Yunnan Observatory, National Astronomical Observatories, Chinese Academy of Sciences, PO Box 110, Kunming 650011, PR China
²²  United Laboratory of Optical Astronomy, Chinese Academy of Sciences, Beijing, PR China
²³  Yunnan Astrophysics Center, Yunnan University, Kunming 650091, PR China
²⁴  Department of Natural Sciences, Fayetteville State University, 1200 Murchison Road, Fayetteville, NC 28301, USA
²⁵  Guadarrama Observatory, C/ San Pablo 5, Villalba 28409, Madrid, Spain
²⁶  Department of Physics and Astronomy, Hampden-Sydney College, Hampden-Sydney, VA 23943, USA
²⁷  Istituto Nazionale di Astrofisica (INAF), Telescopio Nazionale Galileo, Roque de los Muchachos Astronomical Observatory, PO Box 565, 38700 Santa Cruz de La Palma, TF, Spain
²⁸  Department of Astronomy, Faculty of Science, Kyoto University, Kyoto, Japan
²⁹  Stazione Astronomica Vallinfreda, Italy
³⁰  Stazione Astronomica Greve in Chianti, Italy
³¹  Dipartimento di Fisica, Università La Sapienza, Piazzale A. Moro 2, 00185 Roma, Italy
³²  Institute for Astrophysical Research, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA
³³  Dipartimento di Fisica Generale, Università di Torino, via P. Giuria 1, 10125 Torino, Italy
³⁴  Nordic Optical Telescope, Roque de los Muchachos Astronomical Observatory, PO Box 474, 38700 Santa Cruz de La Palma, TF, Spain
³⁵  G.A.C.E., Departament d'Astronomia i Astrofísica, Universitat de València, 46071 Paterna-València, Spain

(Received 18 December 2003 / Accepted 8 March 2004 )

Abstract
BL Lacertae has been the target of four observing campaigns by the Whole Earth Blazar Telescope (WEBT) collaboration. In this paper we present light curves obtained by the WEBT from 1994 to 2002, including the last, extended BL Lac 2001 campaign. A total of about 7500 optical observations performed by 31 telescopes from Japan to Mexico have been collected, to be added to the ~ observations of the BL Lac Campaign 2000. All these data allow one to follow the source optical emission behaviour with unprecedented detail. The analysis of the colour indices reveals that the flux variability can be interpreted in terms of two components: longer-term variations occurring on a few-day time scale appear as mildly-chromatic events, while a strong bluer-when-brighter chromatism characterizes very fast (intraday) flares. By decoupling the two components, we quantify the degree of chromatism inferring that longer-term flux changes imply moving along a ~ 0.1 bluer-when-brighter slope in the B-R versus R plane; a steeper slope of ~ 0.4 would distinguish the shorter-term variations. This means that, when considering the long-term trend, the B-band flux level is related to the R-band one according to a power law of index ~ 1.1. Doppler factor variations on a "convex" spectrum could be the mechanism accounting for both the long-term variations and their slight chromatism.

Offprint request: M. Villata, villata@to.astro.it

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