Multifrequency variability of the blazar AO 0235+164

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

¹ 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.