Multifrequency variability of the blazar AO 0235+164 . The WEBT campaign in 2004-2005 and long-term SED analysis
Lee, C. -U.; Ilyin, I.; Lähteenmäki, A.; Umana, G.; Papadakis, I. E.; Villata, M.; Raiteri, C. M.; Larionov, V. M.; Arkharov, A. A.; Carosati, D.; Kurtanidze, O. M.; Nikolashvili, M. G.; Sigua, L. A.; Tornikoski, M.; Kadler, M.; Kerp, J.; Ros, E.; Kraus, A.; Walters, R.; Trigilio, C.; Leto, P.; Buemi, C. S.; Aller, H. D.; Aller, M. F.; Romero, G. E.; Lindfors, E.; Nilsson, K.; Carini, M. T.; Efimova, N. V.; Berdyugin, A.; Fuhrmann, L.; Krichbaum, T. P.; Ungerechts, H.; Pursimo, T.; Witzel, A.; Bach, U.; Smith, S.; Kovalev, Y. Y.; Cellone, S. A.; Pasanen, M.; Boltwood, P.; Takalo, L. O.; Araudo, A. T.; Ohlert, J.; Hagen-Thorn, V. A.; Konstantinova, T. S.; Lanteri, L.; Xilouris, E.; Kovalev, Yu. A.; Mattox, J. R.; Dolci, M.; Ivanidze, R. Z.; Ibrahimov, M. A.; Barnaby, D.; Cool, R.; Holcomb, M.; Impellizzeri, V.; Kapanadze, B. Z.; Napoleone, N.; Poteet, C.; Tröller, M.
Italy, Germany, United States, Uzbekistan, Georgia, Russia, Finland, Canada, South Korea, Argentina, Greece, Spain
Abstract
Aims.A huge multiwavelength campaign targeting the blazar
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.