The unprecedented optical outburst of the quasar 3C 454.3

The WEBT campaign of 2004–2005

¹ INAF, Osservatorio Astronomico di Torino, Italy e-mail: villata@to.astro.it
² Foggy Bottom Observatory, Colgate University, NY, USA
³ Department of Astronomy, University of Michigan, MI, USA
⁴ Institute for Astrophysical Research, Boston University, MA, USA
⁵ Abastumani Astrophysical Observatory, Georgia
⁶ Astrophysikalisches Institut Potsdam, Germany
⁷ Landessternwarte Heidelberg-Königstuhl, Germany
⁸ Harvard-Smithsonian Center for Astrophysics, MA, USA
⁹ Instituto de Astronomía, UNAM, Mexico
¹⁰ INAF, Osservatorio Astronomico di Roma, Italy
¹¹ Tuorla Observatory, Finland
¹² Astronomical Institute, Osaka Kyoiku University, Japan
¹³ Main (Pulkovo) Astronomical Observatory of the Russian Academy of Sciences, Russia
¹⁴ INAF, Osservatorio Astrofisico di Catania, Italy
¹⁵ Astrophysical Institute, Department of Physics and Astronomy, Ohio University, OH, USA
¹⁶ Armenzano Astronomical Observatory, Italy
¹⁷ Agrupació Astronòmica de Sabadell, Spain
¹⁸ Institute of Astronomy, National Central University, Taiwan
¹⁹ Dipartimento di Fisica e Osservatorio Astronomico, Università di Perugia, Italy
²⁰ INAF, Osservatorio Astronomico di Teramo, Italy
²¹ Departamento de Astrofísica, Universidad Complutense, Spain
²² Astronomical Institute, St.-Petersburg State University, Russia
²³ Isaac Newton Institute of Chile, St.-Petersburg Branch, Russia
²⁴ Institute of Theoretical Physics and Astrophysics, Masaryk University, Czech Republic
²⁵ Astronomical Institute, Academy of Sciences of the Czech Republic, Czech Republic
²⁶ Ulugh Beg Astronomical Institute, Academy of Sciences of Uzbekistan, Uzbekistan
²⁷ Department of Astronomy, Kyoto University, Japan
²⁸ Jansky fellow, National Radio Astronomy Observatory, WV, USA
²⁹ Astro Space Center of Lebedev Physical Institute, Russia
³⁰ Max-Planck-Institut für Radioastronomie, Germany
³¹ Division of Astronomy, University of Oulu, Finland
³² Korea Astronomy and Space Science Institute, South Korea
³³ INAF, Istituto di Radioastronomia Sezione di Noto, Italy
³⁴ Metsähovi Radio Observatory, Helsinki University of Technology, Finland
³⁵ Instituto Nacional de Astrofísica, Optica y Electrónica (INAOE), Mexico
³⁶ School of Physics and Astronomy, The University, UK
³⁷ Michael Adrian Observatory, Germany
³⁸ Nordic Optical Telescope, Roque de los Muchachos Astronomical Observatory, TF, Spain
³⁹ Department of Physics, University of Colorado at Denver and Health Sciences Center, CO, USA
⁴⁰ Crimean Astrophysical Observatory, Ukraine
⁴¹ Radio Astronomy Laboratory of Crimean Astrophysical Observatory, Ukraine
⁴² Institut für Theoretische Physik und Astrophysik der Universität Kiel, Germany

Received: 31 January 2006
Accepted: 2 March 2006

Abstract

Context.The radio quasar 3C 454.3 underwent an exceptional optical outburst lasting more than 1 year and culminating in spring 2005. The maximum brightness detected was , which represents the most luminous quasar state thus far observed ().

Aims.In order to follow the emission behaviour of the source in detail, a large multiwavelength campaign was organized by the Whole Earth Blazar Telescope (WEBT).

Methods.Continuous optical, near-IR and radio monitoring was performed in several bands. ToO pointings by the Chandra and INTEGRAL satellites provided additional information at high energies in May 2005.

Results.The historical radio and optical light curves show different behaviours. Until about 2001.0 only moderate variability was present in the optical regime, while prominent and long-lasting radio outbursts were visible at the various radio frequencies, with higher-frequency variations preceding the lower-frequency ones. After that date, the optical activity increased and the radio flux is less variable. This suggests that the optical and radio emissions come from two separate and misaligned jet regions, with the inner optical one acquiring a smaller viewing angle during the 2004-2005 outburst. Moreover, the colour-index behaviour (generally redder-when-brighter) during the outburst suggests the presence of a luminous accretion disc. A huge mm outburst followed the optical one, peaking in June-July 2005. The high-frequency (37-43 GHz) radio flux started to increase in early 2005 and reached a maximum at the end of our observing period (end of September 2005). VLBA observations at 43 GHz during the summer confirm the brightening of the radio core and show an increasing polarization. An exceptionally bright X-ray state was detected in May 2005, corresponding to the rising mm flux and suggesting an inverse-Compton nature of the hard X-ray spectrum.

Conclusions.A further multifrequency monitoring effort is needed to follow the next phases of this unprecedented event.