The high activity of 3C 454.3 in autumn 2007. Monitoring by the WEBT during the AGILE detection

Abstract

The quasar-type blazar 3C 454.3 underwent a phase of high activity in summer and autumn 2007, which was intensively monitored in the radio-to-optical bands by the Whole Earth Blazar Telescope (WEBT). The gamma-ray satellite AGILE detected this source first in late July, and then in November-December 2007. In this letter we present the multifrequency data collected by the WEBT and collaborators during the second AGILE observing period, complemented by a few contemporaneous data from 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. 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 in 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. In the considered period, the source varied by 2.6 mag in a couple of 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 hours was detected, followed by a steep decrease of about 1.2 mag in 1 hour. 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 going towards fainter states, suggesting a UV excess likely due to the thermal emission from the accretion disc.

5 pages, 4 figures, accepted for publication in Astronomy & Astrophysics Letters

References in corpus (4)

• Multifrequency monitoring of the blazar 0716+714 during the GASP-WEBT-AGILE campaign of 2007
• AGILE detection of a strong gamma-ray flare from the blazar 3C 454.3
• The WEBT Campaign on the Blazar 3C279 in 2006
• Radio-to-UV monitoring of AO 0235+164 by the WEBT and Swift during the 2006--2007 outburst

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• Multiwavelength observations of 3C 454.3. I. The AGILE 2007 November campaign on the "Crazy Diamond"
• Long term variability and correlation study of the blazar 3C 454.3 in radio, NIR and optical wavebands
• Clues on High Energy Emission Mechanism from Blazar 3C 454.3 during 2015 August Flare
• Gamma-Ray Flares in Long Term Light Curve of 3C 454.3
• Blazar variability with the Vera C. Rubin Legacy Survey of Space and Time (LSST)
• Optical variability of the blazar 3C 371: from minute to year timescales
• Spectral index-mass accretion rate correlation and evaluation of black hole masses in AGNs 3C~454.3 and M87
• Modeling the spectral-energy-distribution of 3C 454.3 in a "flat" broad-line-region scenario
• Revisiting radio variability of the blazar 3C 454.3
• Correlations between $\gamma$-ray luminosity and magnetization of the jet as well as relativistic electron injection power:cases for Mrk 421, 3C 454.3 and 3C 279
• Multiband Optical Variability of the Blazar 3C 454.3 on Diverse Timescales