Multifrequency variability of the blazar AO0235+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.; TORNIKOSK,I M.; BOLTWOOD, P.; LEE, C.-U.; ALLER, M. F.; ROMERO, G. E.; ALLER, H. D.; ARAUDO, A. T.; ARKHAROV, A. A.; BACH, U.; BARNABY, D.; BERDYUGIN, A.; BUEMI, C. S.; CARINI, M. T.; CAROSATI, D.; CELLONE, S. A.; COOL, R.; DOLCI, M.; EFIMOVA, N. V.; FUHRMANN, L.; HAGEN-THORN, V. A.; HOLCOMB, M.; ILYIN, I.; IMPELLIZZERI, V.; IVANIDZE, R. Z.; KAPANADZE, B. Z.; KERP, J.; KONSTANTINOVA, T. S.; KOVALEV, Y. Y.; KOVALEV, YU. A.; KRAUS, A.; KRICHBAUM, T. P.; LÄHTEENMÄKI, A.; LANTERI, L.; LETO, P.; LINDFORS, E.; MATTOX, J. R.; NAPOLEONE, N.; NIKOLASHVILI, M. G.; NILSSON, K.; OHLERT, J.; PAPADAKIS, I. E.; PASANEN, M.; POTEET, C.; PURSIMO, T.; ROS, E.; SIGUA, L. A.; SMITH, S.; TAKALO, L. O.; TRIGILIO, C.; TRÖLLER, M.; UMANA, G.; UNGERECHTS, H.; WALTERS, R.; WITZEL, A.; XILOURIS, E.

ASTRONOMY AND ASTROPHYSICS

The European Southern Observatory - EDP Sciences

Lugar: Paris; 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.