A new activity phase of the blazar 3C 454.3^*

Multifrequency observations by the WEBT and XMM-Newton in 2007–2008

¹ INAF – Osservatorio Astronomico di Torino, Italy e-mail: raiteri@oato.inaf.it
² Astron. Inst., St.-Petersburg State Univ., Russia
³ Pulkovo Observatory, St. Petersburg, Russia
⁴ Harvard-Smithsonian Center for Astroph., Cambridge, MA, USA
⁵ Institute of Astronomy, National Central University, Taiwan
⁶ Abastumani Astrophysical Observatory, Georgia
⁷ Department of Astronomy, University of Michigan, MI, USA
⁸ Department of Physics and Astronomy, Ohio Univ., OH, USA
⁹ Osservatorio Astronomico della Regione Autonoma Valle d'Aosta, Italy
¹⁰ Inst. of Theor. Phys. and Astroph., Masaryk Univ., Czech Republic
¹¹ Metsähovi Radio Obs., Helsinki Univ. of Technology, Finland
¹² Korea Astronomy and Space Science Institute, South Korea
¹³ Tuorla Observatory, Univ. of Turku, Piikkiö, Finland
¹⁴ Michael Adrian Observatory, Trebur, Germany
¹⁵ IESL, FORTH, Heraklion, Crete, Greece
¹⁶ Physics Department, University of Crete, Greece
¹⁷ Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain
¹⁸ Max-Planck-Institut für Radioastronomie, Bonn, Germany
¹⁹ Inst. of Astronomy, Bulgarian Academy of Sciences, Sofia, Bulgaria
²⁰ INAF – Osservatorio Astrofisico di Catania, Italy
²¹ Armenzano Astronomical Observatory, Italy
²² Université de Bordeaux, Observatoire Aquitain des Sciences de l'Univers, Floirac, France
²³ CNRS, Laboratoire d'Astrophysique de Bordeaux, UMR 5804, Floirac, France
²⁴ Agrupació Astronòmica de Sabadell, Spain
²⁵ ARIES, Manora Peak, Nainital, India
²⁶ School of Cosmic Physics, Dublin Institute For Advanced Studies, Ireland
²⁷ Institute for Astrophysical Research, Boston University, MA, USA
²⁸ Sofia University, Bulgaria
²⁹ INAF – Istituto di Radioastronomia, Sezione di Noto, Italy
³⁰ School of Physics, University College Dublin, Ireland
³¹ Dept. of Phys. “La Sapienza” Univ, Roma, Italy
³² INAF – Osservatorio Astronomico di Roma, Italy
³³ N. Copernicus Observatory and Planetarium in Brno, Czech Republic
³⁴ INAF – Osservatorio Astronomico di Trieste, Italy
³⁵ Nordic Optical Telescope, Santa Cruz de La Palma, Spain
³⁶ Dept. of Phys., Univ. of Colorado Denver, Denver, CO USA
³⁷ Cork Institute of Technology, Cork, Ireland
³⁸ INAF, IASF-Roma, Italy
³⁹ INAF, IASF-Milano, Italy
⁴⁰ Radio Astronomy Lab. of Crimean Astrophysical Observatory, Ukraine
⁴¹ Instituto de Radioastronomía Millimétrica, Granada, Spain

Received: 27 August 2008
Accepted: 2 October 2008

Abstract

Aims. The Whole Earth Blazar Telescope (WEBT) consortium has been monitoring the blazar 3C 454.3 from the radio to the optical bands since 2004 to study its emission variability properties.

Methods. We present and analyse the multifrequency results of the 2007-2008 observing season, including XMM-Newton observations and near-IR spectroscopic monitoring, and compare the recent emission behaviour with the past one. The historical mm light curve is presented here for the first time.

Results. In the optical band we observed a multi-peak outburst in July-August 2007, and other faster events in November 2007-February 2008. During these outburst phases, several episodes of intranight variability were detected. A mm outburst was observed starting from mid 2007, whose rising phase was contemporaneous to the optical brightening. A slower flux increase also affected the higher radio frequencies, the flux enhancement disappearing below 8 GHz. The analysis of the optical-radio correlation and time delays, as well as the behaviour of the mm light curve, confirm our previous predictions, suggesting that changes in the jet orientation likely occurred in the last few years. The historical multiwavelength behaviour indicates that a significant variation in the viewing angle may have happened around year 2000. Colour analysis confirms a general redder-when-brighter trend, which reaches a “saturation” at R ~ 14 and possibly turns into a bluer-when-brighter trend in bright states. This behaviour is due to the interplay of different emission components, the synchrotron one possibly being characterised by an intrinsically variable spectrum. All the near-IR spectra show a prominent Hα emission line (EW_obs = 50-120 Å), whose flux appears nearly constant, indicating that the broad line region is not affected by the jet emission. We show the broad-band SEDs corresponding to the epochs of the XMM-Newton pointings and compare them to those obtained at other epochs, when the source was in different brightness states. A double power-law fit to the EPIC spectra including extra absorption suggests that the soft-X-ray spectrum is concave, and that the curvature becomes more pronounced as the flux decreases. This connects fairly well with the UV excess, which becomes more prominent with decreasing flux. The most obvious interpretation implies that, as the beamed synchrotron radiation from the jet dims, we can see both the head and the tail of the big blue bump. The X-ray flux correlates with the optical flux, suggesting that in the inverse-Compton process either the seed photons are synchrotron photons at IR-optical frequencies or the relativistic electrons are those that produce the optical synchrotron emission. The X-ray radiation would thus be produced in the jet region from where the IR-optical emission comes.