The WEBT campaign to observe AO 0235+16 in the 2003–2004 observing season

Results from radio-to-optical monitoring and XMM-Newton observations

¹ INAF - Osservatorio Astronomico di Torino, Italy, e-mail: raiteri@to.astro.it
² Ulugh Beg Astronomical Inst., Uzbekistan
³ Astronomical Inst., St.-Petersburg State Univ., Russia
⁴ Isaac Newton Inst. of Chile, St. Petersburg Branch
⁵ Max-Planck-Institut für Radioastronomie, Germany
⁶ Dept. of Astronomy, Univ. of Michigan, USA
⁷ National Radio Astronomy Observatory, Green Bank, USA
⁸ Astro Space Center of Lebedev Physical Inst., Russia
⁹ Tuorla Astronomical Observatory, Finland
¹⁰ IESL, FORTH, Greece
¹¹ Physics Dept., Univ. of Crete, Greece
¹² Nordic Optical Telescope, Santa Cruz de La Palma, Spain
¹³ Inst. Argentino de Radioastronomía, Argentina
¹⁴ Metsähovi Radio Observatory, Helsinki Univ. of Technology, Finland
¹⁵ Pulkovo Observatory, Russia
¹⁶ Dept. of Physics & Astronomy, Western Kentucky Univ., USA
¹⁷ Dept. of Physics and Astronomy, Ohio Univ., USA
¹⁸ Observatory, Univ. of Helsinki, Finland
¹⁹ Armenzano Astronomical Observatory, Italy
²⁰ Facultad de Ciencias Astronómicas y Geofísicas, Argentina
²¹ Dept. de Física, Escuela Politécnica Superior, Univ. de Jaén, Spain
²² Dept. of Physics & Astronomy, Univ. of Hawaii, USA
²³ INAF - Osservatorio Astronomico di Roma, Italy
²⁴ INAF - Osservatorio Astronomico di Collurania Teramo, Italy
²⁵ Osservatorio Astronomico, Univ. di Perugia, Italy
²⁶ Korea Astronomy Observatory, Korea
²⁷ Inst. for Astrophysical Research, Boston Univ., USA
²⁸ Abastumani Observatory, Georgia
²⁹ Special Astrophysical Observatory, Karachaevo-Cherkessia, Russia
³⁰ Dipartimento di Fisica, Univ. di Roma “La Sapienza”, Italy
³¹ Michael Adrian Observatory, Trebur, Germany
³² Landessternwarte Heidelberg, Germany
³³ Indian Inst. of Astrophysics, Bangalore, India
³⁴ Agrupació Astronòmica de Sabadell, Spain
³⁵ Dept. of Applied Physics & Instrumentation, Cork Inst. of Technology, Ireland
³⁶ SARA Observatory, Florida International Univ., USA
³⁷ Inst. of Astronomy and Astrophysics, National Observatory of Athens, Greece

Received: 17 December 2004
Accepted: 17 February 2005

Abstract

A multiwavelength campaign to observe the BL Lac object AO 0235+16 () was set up by the Whole Earth Blazar Telescope (WEBT) collaboration during the observing seasons 2003–2004 and 2004–2005, involving radio, near-IR and optical photometric monitoring, VLBA monitoring, optical spectral monitoring, and three pointings by the XMM-Newton satellite. Here we report on the results of the first season, which involved the participation of 24 optical and near-IR telescopes and 4 radio telescopes, as well as the first XMM-Newton pointing, which occurred on January 18–19, 2004. Unpublished data from previous epochs were also collected (from 5 optical-NIR and 3 radio telescopes), in order to fill the gap between the end of the period presented in Raiteri et al. (2001) and the start of the WEBT campaign. The contribution of the southern AGN, 2 arcsec distant from the source, is taken into account. It is found to especially affect the blue part of the optical spectrum when the source is faint. In the optical and near-IR the source has been very active in the last 3 years, although it has been rather faint most of the time, with noticeable variations of more than a magnitude over a few days. In contrast, in the radio bands it appears to have been “quiescent” since early 2000. The major radio (and optical) outburst predicted to peak around February–March 2004 (with a six month uncertainty) has not occurred yet. When comparing our results with the historical light curves, two different behaviours seem to characterize the optical outbursts: only the major events present a radio counterpart. The X-ray spectra obtained by the three EPIC detectors are well fitted by a power law with extra-absorption at ; the energy index in the 0.2–10 keV range is well constrained: ± 0.028 and the 1 keV flux density is 0.311 ± . The analysis of the X-ray light curves reveals that no significant variations occurred during the pointing. In contrast, simultaneous dense radio monitoring with the 100 m telescope at Effelsberg shows a ~2–3% flux decrease in 6–7 h, which, if intrinsic, would imply a brightness temperature well above the Compton limit and hence a lower limit to the Doppler factor . We construct the broad-band spectral energy distribution of January 18–19, 2004 with simultaneous radio data from Effelsberg, optical data from the Nordic Optical Telescope (NOT), optical–UV data from the Optical Monitor onboard XMM-Newton, and X-ray data by the EPIC instruments. Particular care is taken to correct data for extinction due to both the Milky Way and the absorber. The resulting SED suggests the existence of a bump in the UV spectral region.