C. M. Raiteri, et al. (incl. M. Böttcher,
B. Ragozzine, and K. Uckert),
Astronomy & Astrophysics, 485, L17 (2008)
Context. The quasar-type blazar 3C 3C454.3 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 Astro-rivelatore Gamma a
Immagini LEggero (AGILE) detected this source first in late July,
and then in November - December 2007.
Aims. 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 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 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.
Results. 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.,
ingranight) 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.
Conclusions. Once the AGILE data are completely analysed, the
low-frequency observations presented in this letter will offer a
formidable tool to investigate the optical - gamma flux correlations,
i.e., the relationship between the synchrotron and inverse-Compton
emission components.