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 gamma-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 diffcult 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-gamma flux correlations, i.e. the relationship between the synchrotron and inverse-Compton emission components.
The high activity of 3C 454.3 in autumn 2007 - Monitoring by the WEBT during the AGILE detection / Raiteri CM; Villata M; Chen WP; Hsiao WS; Kurtanidze OM; Nilsson K; Larionov VM; Gurwell MA; Agudo I; Aller HD; Aller MF; Angelakis E; Arkharov AA; Bach U; Bottcher M; Buemi CS; Calcidese P; Charlot P; DAmmando F; Donnarumma I; Forne E; Frasca A; Fuhrmann L; Gomez JL; Hagen-Thorn VA; Jorstad SG; Kimeridze GN; Krichbaum TP; Lahteenmaki A; Lanteri L; Latev G; Le Campion JF; Lee CU; Leto P; Lin HC; Marchili N; Marilli E; Marscher AP; Nesci R; Nieppola E; Nikolashvili MG; Ohlert J; Ovcharov E; Principe D; Pursimo T; Ragozzine B; Sadun AC; Sigua LA; Smart RL; Strigachev A; Takalo LO; Tavani M; Thum C; Tornikoski M; Trigilio C; Uckert K; Umana G; Valcheva A; Vercellone S; Volvach A; Wiesemeyer H. - In: ASTRONOMY & ASTROPHYSICS. - ISSN 0004-6361. - STAMPA. - 485:2(2008), pp. 17-20. [10.1051/0004-6361:200809995]
The high activity of 3C 454.3 in autumn 2007 - Monitoring by the WEBT during the AGILE detection
D'AMMANDO, FILIPPO;
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 gamma-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 diffcult 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-gamma flux correlations, i.e. the relationship between the synchrotron and inverse-Compton emission components.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
