Radio-to-UV monitoring of AO 0235+164 by the WEBT and Swift during the 2006–2007 outburst

¹ INAF, Osservatorio Astronomico di Torino, Italy e-mail: raiteri@oato.inaf.it
² Astron. Inst., St.-Petersburg State Univ., Russia
³ Pulkovo Observatory, St. Petersburg, Russia
⁴ Department of Astronomy, University of Michigan, MI, USA
⁵ Max-Planck-Institut für Radioastronomie, Bonn, Germany
⁶ Harvard-Smithsonian Center for Astroph., Cambridge, MA, USA
⁷ Abastumani Astrophysical Observatory, Georgia
⁸ Metsähovi Radio Obs., Helsinki Univ. of Technology, Finland
⁹ Tuorla Observatory, Univ. of Turku, Piikkiö, Finland
¹⁰ Radio Astron. Lab. of Crimean Astroph. Observatory, Ukraine
¹¹ Inst. of Astron., Bulgarian Acad. of Sciences, Sofia, Bulgaria
¹² Department of Physics and Astronomy, Ohio Univ., OH, USA
¹³ INAF, Osservatorio Astrofisico di Catania, Italy
¹⁴ Oss. Astronomico della Regione Autonoma Valle d'Aosta, Italy
¹⁵ New Millennium Observatory, Italy
¹⁶ INAF, Osservatorio Astronomico di Roma, Italy
¹⁷ INAF, Osservatorio Astronomico di Collurania Teramo, Italy
¹⁸ Center for Astrophysics, Guangzhou University, PR China
¹⁹ Agrupació Astronòmica de Sabadell, Spain
²⁰ INAF, IASF-Bologna, Italy
²¹ YNAO, Chinese Academy of Sciences, Kunming, PR China
²² NASA/Goddard Space Flight Center, Greenbelt, Maryland, USA
²³ Inst. of Astronomy, BAN, Bulgaria
²⁴ Max-Planck-Institut für Radioastronomie, 53121 Bonn, Germany
²⁵ Korea Astronomy and Space Science Institute, South Korea
²⁶ INAF, Istituto di Radioastronomia, Sezione di Noto, Italy
²⁷ Greve Observatory, Italy
²⁸ Dept. of Phys., “La Sapienza” Univ., Roma, Italy
²⁹ Michael Adrian Observatory, Trebur, Germany
³⁰ Hankasalmi Observatory, Finland
³¹ Sofia University, Bulgaria
³² Jakokoski Observatory, Univ. of Joensuu, Finland
³³ Nordic Optical Telescope, Santa Cruz de La Palma, Spain
³⁴ Osaka University, Japan

Received: 9 November 2007
Accepted: 6 December 2007

Abstract

Context.The blazar AO 0235+164 was claimed to show a quasi-periodic behaviour in the radio and optical bands in the past, with the main outbursts repeating every 5–6 years. However, the predicted 2004 outburst did not occur, and further analysis suggested a longer time scale, according to which the next event would have occurred in the 2006–2007 observing season. Moreover, an extra emission component contributing to the UV and soft X-ray flux was detected, whose nature is not yet clear. An optical outburst was observed in late 2006–early 2007, which triggered a Whole Earth Blazar Telescope (WEBT) campaign as well as target of opportunity (ToO) observations by the Swift satellite.

Aims.In this paper, we present the radio-to-optical data taken by the WEBT together with the UV data acquired by the UltraViolet and Optical Telescope (UVOT) instrument onboard Swift to investigate both the outburst behaviour at different wavelengths and the nature of the extra emission component.

Methods.Multifrequency light curves have been assembled with data from 27 observatories; optical and UV fluxes have been cleaned from the contamination of the southern active galactic nucleus (AGN). We have analysed spectral energy distributions at different epochs, corresponding to different brightness states; extra absorption by the foreground galaxy has been taken into account.

Results.We found the optical outburst to be as strong as the big outbursts of the past: starting from late September 2006, a brightness increase of ~5 mag led to the outburst peak in February 19–21, 2007. We also observed an outburst at mm and then at cm wavelengths, with an increasing time delay going toward lower frequencies during the rising phase. Cross-correlation analysis indicates that the 1 mm and 37 GHz flux variations lagged behind the R-band ones by about 3 weeks and 2 months, respectively. These short time delays suggest that the corresponding jet emitting regions are only slightly separated and/or misaligned. In contrast, during the outburst decreasing phase the flux faded contemporaneously at all cm wavelengths. This abrupt change in the emission behaviour may suggest the presence of some “shutdown” mechanism of intrinsic or geometric nature. The behaviour of the UV flux closely follows the optical and near-IR one. By separating the synchrotron and extra component contributions to the UV flux, we found that they correlate, which suggests that the two emissions have a common origin.