Multifrequency variability of the blazar AO 0235+164. The WEBT campaign in 2004-2005 and long-term SED analysis

RAITERI, C. M.; VILLATA, M.; KADLER, M.; IBRAHIMOV, M. A.; KURTANIDZE, O. M.; LARIONOV, V. M.; TORNIKOSKI, M.; BOLTWOOD, P.; LEE, C.-U.; ALLER, M. F.; ROMERO, G.E.; AND 50 MORE AUTHORS

ASTRONOMY AND ASTROPHYSICS

EDP Sciences

Lugar: Les Ulis Cedex A; Año: 2006 vol. 459 p. 731 - 743

0004-6361

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.