A&A 459, 731-743 (2006)
DOI: 10.1051/0004-6361:20065744

Multifrequency variability of the blazar AO 0235+164

The WEBT campaign in 2004-2005 and long-term SED analysis

C. M. Raiteri¹, M. Villata¹, M. Kadler^{2, 3}, M. A. Ibrahimov⁴, O. M. Kurtanidze⁵, V. M. Larionov⁶, M. Tornikoski⁷, P. Boltwood⁸, C.-U. Lee⁹, M. F. Aller¹⁰, G. E. Romero^{11, 12}, H. D. Aller¹⁰, A. T. Araudo^{11, 12}, A. A. Arkharov¹³, U. Bach¹, D. Barnaby¹⁴, A. Berdyugin¹⁵, C. S. Buemi¹⁶, M. T. Carini¹⁴, D. Carosati¹⁷, S. A. Cellone¹², R. Cool¹⁸, M. Dolci¹⁹, N. V. Efimova¹³, L. Fuhrmann^{1, 20}, V. A. Hagen-Thorn⁶, M. Holcomb¹⁴, I. Ilyin²¹, V. Impellizzeri², R. Z. Ivanidze⁵, B. Z. Kapanadze⁵, J. Kerp²², T. S. Konstantinova⁶, Y. Y. Kovalev^{23, 24, 2}, Yu. A. Kovalev²⁴, A. Kraus², T. P. Krichbaum², A. Lähteenmäki⁷, L. Lanteri¹, P. Leto²⁵, E. Lindfors¹⁵, J. R. Mattox²⁶, N. Napoleone²⁷, M. G. Nikolashvili⁵, K. Nilsson¹⁵, J. Ohlert²⁸, I. E. Papadakis^{29, 30}, M. Pasanen¹⁵, C. Poteet¹⁴, T. Pursimo³¹, E. Ros², L. A. Sigua³², S. Smith¹⁴, L. O. Takalo¹⁵, C. Trigilio¹⁶, M. Tröller⁷, G. Umana¹⁶, H. Ungerechts³³, R. Walters¹⁴, A. Witzel², and E. Xilouris<sup>34

1</sup>  INAF, Osservatorio Astronomico di Torino, 10025 Pino Torinese, Italy
    e-mail: raiteri@to.astro.it
²  Max-Planck-Institut für Radioastronomie, 53121 Bonn, Germany
³  NASA/Goddard Space Flight Center, Code 662, Greenbelt, Maryland 20771, USA
⁴  Ulugh Beg Astron. Inst., Academy of Sciences of Uzbekistan, Tashkent 700052, Uzbekistan
⁵  Abastumani Observatory, 383762 Abastumani, Georgia
⁶  Astron. Inst., St.-Petersburg State Univ., 198504 St.-Petersburg, Russia
⁷  Metsähovi Radio Observatory, Helsinki Univ. of Technology, 02540 Kylmälä, Finland
⁸  1655 Stittsville Main St., Stittsville, Ont., K2S 1N6, Canada
⁹  Korea Astronomy and Space Science Institute, Republic of Korea
¹⁰  Dept. of Astronomy, Univ. of Michigan, Ann Arbor, MI 48109, USA
¹¹  Inst. Argentino de Radioastronomía, 1894 Villa Elisa, Argentina
¹²  Facultad de Ciencias Astronómicas y Geofísicas, UNLP, La Plata, Argentina
¹³  Pulkovo Observatory, St. Petersburg, Russia
¹⁴  Dept. of Physics & Astronomy, Western Kentucky Univ., Bowling Green, KY 42104, USA
¹⁵  Tuorla Observatory, 21500 Piikkiö, Finland
¹⁶  INAF, Osservatorio Astrofisico di Catania, 95123 Catania, Italy
¹⁷  Armenzano Astronomical Observatory, Assisi, Italy
¹⁸  Steward Observatory, 933 N.Cherry Ave. Tucson, AZ 85721, USA
¹⁹  INAF, Osservatorio Astronomico di Collurania Teramo, 64100 Teramo, Italy
²⁰  Dipartimento di Fisica e Osservatorio Astronomico, Università di Perugia, Italy
²¹  Astrophysikalisches Institut Potsdam, 14482 Potsdam, Germany
²²  Argelander Institut für Astronomie, Universität Bonn, 53121 Bonn, Germany
²³  National Radio Astronomy Observatory, Green Bank, WV 24944, USA
²⁴  Astro Space Center of Lebedev Physical Inst., 117997 Moscow, Russia
²⁵  INAF, Istituto di Radioastronomia, Sezione di Noto, 96017 Noto, Italy
²⁶  Dept. of Natural Sciences, Fayetteville State Univ., Fayetteville, NC 28301, USA
²⁷  INAF, Osservatorio Astronomico di Roma, 00040 Monte Porzio Catone, Italy
²⁸  Michael Adrian Observatory, 65468 Trebur, Germany
²⁹  IESL, FORTH, 711 10 Heraklion, Crete, Greece
³⁰  Physics Dept., Univ. of Crete, 710 03 Heraklion, Crete, Greece
³¹  Nordic Optical Telescope, 38700 Santa Cruz de La Palma, Spain
³²  Akhaltsikhe branch of the Tbilisi State University, Georgia
³³  IRAM, Avd. Div. Pastora 7NC, 18012 Granada, Spain
³⁴  Inst. of Astronomy and Astrophysics, National Observatory of Athens, 11810 Athens, Greece

(Received 1 June 2006 / Accepted 16 August 2006 )

Abstract
Aims.A huge multiwavelength campaign targeting the blazar AO 0235+164 was organized by the Whole Earth Blazar Telescope (WEBT) in 2003-2005 to study the variability properties of the source.
Methods.Monitoring observations were carried out at cm and mm wavelengths, and in the near-IR and optical bands, while three pointings by the XMM-Newton satellite provided information on the X-ray and UV emission.
Results.We present the data acquired during the second observing season, 2004-2005, by 27 radio-to-optical telescopes. The ~2600 data points collected allow us to trace the low-energy behaviour of the source in detail, revealing an increased near-IR and optical activity with respect to the previous season. Increased variability is also found at the higher radio frequencies, down to ~15 GHz, but not at the lower ones. While the X-ray (and optical) light curves obtained during the XMM-Newton pointings reveal no significant short-term variability, the simultaneous intraday radio observations with the 100 m telescope at Effelsberg show flux-density changes at 10.5 GHz, which are more likely due to a combination of intrinsic and extrinsic processes.
Conclusions.The radio (and optical) outburst predicted to peak around February-March 2004 on the basis of the previously observed 5-6 yr quasi-periodicity did not occur. The analysis of the optical light curves reveals now a longer characteristic time scale of variability of ~8 yr, which is also present in the radio data. The spectral energy distributions corresponding to the XMM-Newton observations performed during the WEBT campaign are compared with those pertaining to previous pointings of X-ray satellites. Bright, soft X-ray spectra can be described in terms of an extra component, which appears also when the source is faint through a hard UV spectrum and a curvature of the X-ray spectrum. Finally, there might be a correlation between the X-ray and optical bright states with a long time delay of about 5 yr, which would require a geometrical interpretation.

Key words: galaxies: active -- galaxies: BL Lacertae objects: general -- galaxies: BL Lacertae objects: individual: AO 0235+164 -- galaxies: jets -- galaxies: quasars: general

© ESO 2006

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