The long-lasting activity of 3C 454.3

GASP-WEBT and satellite observations in 2008–2010 ^⋆,⋆⋆

¹ INAF, Osservatorio Astronomico di Torino, Italy
e-mail: raiteri@oato.inaf.it
² Department of Astronomy, University of Michigan, MI, USA
³ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
⁴ Abastumani Observatory, Mt. Kanobili, Abastumani, Georgia
⁵ Aalto University Metsähovi Radio Observatory, Kylmälä, Finland
⁶ Astron. Inst., St.-Petersburg State Univ., Russia
⁷ Pulkovo Observatory, St.-Petersburg, Russia
⁸ Isaac Newton Institute of Chile, St.-Petersburg Branch, Russia
⁹ INAF-IASF Palermo, Italy
¹⁰ Instituto de Astrofísica de Andalucía, CSIC, Granada, Spain
¹¹ Institute for Astrophysical Research, Boston University, MA, USA
¹² Max-Planck-Institut für Radioastronomie, Bonn, Germany
¹³ Instituto de Astronomía, Universidad Nacional Autónoma de México, Mexico
¹⁴ Tuorla Observatory, Univ. of Turku, Piikkiö, Finland
¹⁵ Astrophysical Inst., Department of Physics and Astronomy, Ohio Univ., Athens OH, USA
¹⁶ INAF, Osservatorio Astrofisico di Catania, Italy
¹⁷ Osservatorio Astronomico della Regione Autonoma Valle d’ Aosta, Italy
¹⁸ EPT Observatories, Tijarafe, La Palma, Spain
¹⁹ INAF, TNG Fundación Galileo Galilei, La Palma, Spain
²⁰ Institut de Ciències de l’Espai (CSIC-IEEC), Spain
²¹ Agrupació Astronòmica de Sabadell, Spain
²² GraduateInstitute of Astronomy, National Central University, Taiwan
²³ ZAH, Landessternwarte Heidelberg-Königstuhl, Germany
²⁴ Lulin Observatory, National Central University, Taiwan
²⁵ School of Cosmic Physics, Dublin Institute For Advanced Studies, Ireland
²⁶ Department of Physics, National Taiwan University, Taipei, Taiwan
²⁷ Circolo Astrofili Talmassons, Italy
²⁸ Department of Physics, Purdue University, West Lafayette, IN, USA
²⁹ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Piikkiö, Finland
³⁰ Facultad de Ciencias Espaciales, Univ. Nacional Autonoma de Honduras, Tegucigalpa M.D.C., Honduras
³¹ Department of Physics, Univ. of Colorado Denver, CO, USA
³² Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD, USA
³³ Galaxy View Observatory, Sequim, Washington, USA
³⁴ Lowell Observatory, Flagstaff, AZ, USA
³⁵ Radio Astronomy Laboratory of Crimean Astrophysical Observatory, Ukraine

Received: 5 April 2011
Accepted: 24 June 2011

Abstract

Context. The blazar 3C 454.3 is one of the most active sources from the radio to the γ-ray frequencies observed in the past few years.

Aims. We present multiwavelength observations of this source from April 2008 to March 2010. The radio to optical data are mostly from the GASP-WEBT, UV and X-ray data from Swift, and γ-ray data from the AGILE and Fermi satellites. The aim is to understand the connection among emissions at different frequencies and to derive information on the emitting jet.

Methods. Light curves in 18 bands were carefully assembled to study flux variability correlations. We improved the calibration of optical-UV data from the UVOT and OM instruments and estimated the Lyα flux to disentangle the contributions from different components in this spectral region.

Results. The observations reveal prominent variability above 8 GHz. In the optical-UV band, the variability amplitude decreases with increasing frequency due to a steadier radiation from both a broad line region and an accretion disc. The optical flux reaches nearly the same levels in the 2008–2009 and 2009–2010 observing seasons; the mm one shows similar behaviour, whereas the γ and X-ray flux levels rise in the second period. Two prominent γ-ray flares in mid 2008 and late 2009 show a double-peaked structure, with a variable γ/optical flux ratio. The X-ray flux variations seem to follow the γ-ray and optical ones by about 0.5 and 1 d, respectively.

Conclusions. We interpret the multifrequency behaviour in terms of an inhomogeneous curved jet, where synchrotron radiation of increasing wavelength is produced in progressively outer and wider jet regions, which can change their orientation in time. In particular, we assume that the long-term variability is due to this geometrical effect. By combining the optical and mm light curves to fit the γ and X-ray ones, we find that the γ (X-ray) emission may be explained by inverse-Comptonisation of synchrotron optical (IR) photons by their parent relativistic electrons (SSC process). A slight, variable misalignment between the synchrotron and Comptonisation zones would explain the increased γ and X-ray flux levels in 2009–2010, as well as the change in the γ/optical flux ratio during the outbursts peaks. The time delays of the X-ray flux changes after the γ, and optical ones are consistent with the proposed scenario.