Letter to the Editor

The radio delay of the exceptional 3C 454.3 outburst^*

Follow-up WEBT observations in 2005–2006

¹ INAF, Osservatorio Astronomico di Torino, Italy e-mail: villata@oato.inaf.it
² Department of Astronomy, University of Michigan, MI, USA
³ Max-Planck-Institut für Radioastronomie, Germany
⁴ Ulugh Beg Astronomical Institute, Academy of Sciences of Uzbekistan, Uzbekistan
⁵ Astro Space Center of Lebedev Physical Institute, Russia
⁶ National Radio Astronomy Observatory, Green Bank, WV, USA
⁷ Abastumani Astrophysical Observatory, Georgia
⁸ Astrophysikalisches Institut Potsdam, Germany
⁹ Landessternwarte Heidelberg-Königstuhl, Germany
¹⁰ Astronomical Institute, St.-Petersburg State University, Russia
¹¹ Isaac Newton Institute of Chile, St.-Petersburg Branch, Russia
¹² Korea Astronomy and Space Science Institute, South Korea
¹³ INAF, Istituto di Radioastronomia Sezione di Noto, Italy
¹⁴ Metsähovi Radio Observatory, Helsinki University of Technology, Finland
¹⁵ Tuorla Observatory, Finland
¹⁶ Nordic Optical Telescope, Roque de los Muchachos Astronomical Observatory, TF, Spain
¹⁷ Agrupació Astronòmica de Sabadell, Spain
¹⁸ Astronomical Institute, Osaka Kyoiku University, Japan
¹⁹ Radio Astronomy Laboratory of Crimean Astrophysical Observatory, Ukraine
²⁰ INAF, Osservatorio Astrofisico di Catania, Italy
²¹ Moscow State University, Russia
²² Crimean Astrophysical Observatory (CrAO), Ukraine
²³ Dipartimento di Fisica e Osservatorio Astronomico, Università di Perugia, Italy
²⁴ Special Astrophysical Observatory, Russia

Received: 18 December 2006
Accepted: 28 December 2006

Abstract

Context.In spring 2005 the blazar 3C 454.3 was observed in an unprecedented bright state from the near-IR to the hard X-ray frequencies. A mm outburst peaked in June–July 2005, and it was followed by a flux increase at high radio frequencies.

Aims.In this paper we report on multifrequency monitoring by the WEBT aimed at following the further evolution of the outburst in detail. In particular, we investigate the expected correlation and time delays between the optical and radio emissions in order to derive information on the variability mechanisms and jet structure.

Methods.A comparison among the light curves at different frequencies is performed by means of visual inspection and discrete correlation function, and the results are interpreted with a simple model taking into account Doppler factor variations of geometric origin.

Results.The high-frequency radio light curves show a huge outburst starting during the dimming phase of the optical one and lasting more than 1 year. The first phase is characterized by a slow flux increase, while in early 2006 a major flare is observed. The lower-frequency radio light curves show a progressively delayed and fainter event, which disappears below 8 GHz. We suggest that the radio major peak is not physically connected with the spring 2005 optical one, but it is actually correlated with a minor optical flare observed in October–November 2005. This interpretation involves both an intrinsic and a geometric mechanism. The former is represented by disturbances travelling down the emitting jet, the latter being due to the curved-jet motion, with the consequent differential changes of viewing angles of the different emitting regions.