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Issue A&A
Volume 438, Number 1, July IV 2005
Page(s) 39 - 53
Section Extragalactic astronomy
DOI 10.1051/0004-6361:20042567



A&A 438, 39-53 (2005)
DOI: 10.1051/0004-6361:20042567

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

Results from radio-to-optical monitoring and XMM-Newton observations
C. M. Raiteri1, M. Villata1, M. A. Ibrahimov2, V. M. Larionov3, 4, M. Kadler5, H .D. Aller6, M. F. Aller6, Y. Y. Kovalev7, 8, L. Lanteri1, K. Nilsson9, I. E. Papadakis10, 11, T. Pursimo12, G. E. Romero13, H. Teräsranta14, M. Tornikoski14, A. A. Arkharov15, D. Barnaby16, A. Berdyugin9, M. Böttcher17, K. Byckling18, M. T. Carini16, D. Carosati19, S. A. Cellone20, S. Ciprini9, J. A. Combi13, 21, S. Crapanzano1, R. Crowe22, A. Di Paola23, M. Dolci24, L. Fuhrmann1, 5, 25, M. Gu26, V. A. Hagen-Thorn3, 4, P. Hakala18, V. Impellizzeri5, S. Jorstad27, J. Kerp5, G. N. Kimeridze28, Yu. A. Kovalev8, A. Kraus5, T. P. Krichbaum5, O. M. Kurtanidze28, A. Lähteenmäki14, E. Lindfors9, M. G. Mingaliev29, R. Nesci30, M. G. Nikolashvili28, J. Ohlert31, M. Orio1, L. Ostorero32, M. Pasanen9, A. Pati33, C. Poteet16, E. Ros5, J. A. Ros34, P. Shastri33, L. A. Sigua28, A. Sillanpää9, N. Smith35, L. O. Takalo9, G. Tosti25, A. Vasileva3, S. J. Wagner32, R. Walters16, J. R. Webb36, W. Wills16, A. Witzel5 and E. Xilouris37

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 (z=0.94) 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 z=0.524; the energy index in the 0.2-10 keV range is well constrained: $\alpha=0.645$ $\pm$ 0.028 and the 1 keV flux density is 0.311 $\pm$ $0.008~\rm\mu Jy$. 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 $\delta \ga 46$. 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 z=0.524 absorber. The resulting SED suggests the existence of a bump in the UV spectral region.


Key words: galaxies: active -- BL Lacertae objects: general -- BL Lacertae objects: individual: AO 0235+16

SIMBAD Objects



© ESO 2005


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