Letter to the Editor

The high activity of 3C 454.3 in autumn 2007^*

Monitoring by the WEBT during the AGILE detection

¹ INAF, Osservatorio Astronomico di Torino, Italy e-mail: [raiteri;villata]@oato.inaf.it
² Institute of Astronomy, National Central University, Taiwan
³ Abastumani Astrophysical Observatory, Georgia
⁴ Tuorla Observatory, Univ. of Turku, Piikkiö, Finland
⁵ Astron. Inst., St.-Petersburg State Univ., Russia
⁶ Pulkovo Observatory, St. Petersburg, Russia
⁷ Harvard-Smithsonian Center for Astroph., Cambridge, MA, USA
⁸ Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain
⁹ Department of Astronomy, University of Michigan, MI, USA
¹⁰ Max-Planck-Institut für Radioastronomie, Bonn, Germany
¹¹ Department of Physics and Astronomy, Ohio Univ., OH, USA
¹² INAF, Osservatorio Astrofisico di Catania, Italy
¹³ Osservatorio Astronomico della Regione Autonoma Valle d'Aosta, Italy
¹⁴ Lab. d'Astrophysique, Université Bordeaux 1, CNRS, Floirac, France
¹⁵ INAF, IASF-Roma, Italy
¹⁶ Agrupació Astronómica de Sabadell, Spain
¹⁷ Institute for Astrophysical Research, Boston University, MA, USA
¹⁸ Metsähovi Radio Obs., Helsinki Univ. of Technology TKK, Finland
¹⁹ Sofia University, Bulgaria
²⁰ Korea Astronomy and Space Science Institute, South Korea
²¹ INAF, Istituto di Radioastronomia, Sezione di Noto, Italy
²² Dept. of Phys. “La Sapienza” Univ, Roma, Italy
²³ Michael Adrian Observatory, Trebur, Germany
²⁴ Nordic Optical Telescope, Santa Cruz de La Palma, Spain
²⁵ Dept. of Phys., Univ. of Colorado, Denver, USA
²⁶ Inst. of Astronomy, Bulgarian Academy of Sciences, Sofia, Bulgaria
²⁷ Institut de Radio Astronomie Millimétrique, Grenoble, France
²⁸ INAF, IASF-Milano, Italy
²⁹ Radio Astronomy Lab. of Crimean Astrophysical Observatory, Ukraine
³⁰ Instituto de Radioastronomía Millimétrica, Granada, Spain

Received: 17 April 2008
Accepted: 9 May 2008

Abstract

Context. The quasar-type blazar 3C 454.3 underwent a phase of high activity in summer and autumn 2007 that was intensively monitored in the radio-to-optical bands by the Whole Earth Blazar Telescope (WEBT). The γ-ray satellite Astro-rivelatore Gamma a Immagini LEggero (AGILE) detected this source first in late July, and then in November–December 2007.

Aims. We present the multifrequency data collected by the WEBT and collaborators during the second AGILE observing period, complemented by contemporaneous data from the UltraViolet and Optical Telescope (UVOT) onboard the Swift satellite. The aim is to trace in detail the behaviour of the synchrotron emission from the blazar jet, and to investigate the contribution from the thermal emission component.

Methods. Optical data from about twenty telescopes have been homogeneously calibrated and carefully assembled to construct an R-band light curve containing about 1340 data points over 42 days. This extremely well-sampled optical light curve allows us to follow the dramatic flux variability of the source in detail. In addition, we show radio-to-UV spectral energy distributions (SEDs) at different epochs, which represent different brightness levels.

Results. In the considered period, the source varied by 2.6 mag over two weeks in the R band. Many episodes of fast (i.e. intranight) variability were observed, most notably on December 12, when a flux increase of about 1.1 mag in 1.5 h was detected, followed by a steep decrease of about 1.2 mag in 1 h. The contribution by the thermal component is difficult to assess, due to the uncertainties in the Galactic, and possibly also intrinsic, extinction in the UV band. However, polynomial fitting of radio-to-UV SEDs reveals an increasing spectral bending towards fainter states, suggesting a UV excess likely due to the thermal emission from the accretion disc.

Conclusions. Once the AGILE data are completely analysed, the low-frequency observations presented in this letter will offer a powerful tool to investigate optical-γ flux correlations, i.e. the relationship between the synchrotron and inverse-Compton emission components.