Swift and infra-red observations of the blazar 3C 454.3 during the giant X-ray flare of May 2005

¹ ASI Science Data Center, ASDC c/o ESRIN, via G. Galilei, 00044 Frascati, Italy e-mail: paolo.giommi@asi.it
² Agenzia Spaziale Italiana, Unitá Osservazione dell'Universo, Italy
³ UCL, Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁴ INAF – Osservatorio Astronomico di Roma via Frascati 33, 00040 Monteporzio, Italy
⁵ Department of Astronomy and Astrophysics, Pennsylvania State University, USA
⁶ Dipartimento di Fisica, Universitá di Perugia, via A. Pascoli, Perugia, Italy
⁷ NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
⁸ Dipartimento di Fisica, Università “La Sapienza”, P.le A. Moro 2, 00185 Roma, Italy
⁹ INAF – Osservatorio Astronomico di Brera via Bianchi 46, 23807 Merate, Italy

Received: 18 January 2006
Accepted: 13 June 2006

Abstract

We present the results of a series of Swift and quasi simultaneous ground-based infra-red observations of the blazar 3C 454.3 carried out in April-May 2005 when the source was 10 to 30 times brighter than previously observed. We found 3C 454.3 to be very bright and variable at all frequencies covered by our instrumentation. The broad-band Spectral Energy Distribution (SED) shows the usual two-bump shape (in space) with the Infra-red, optical and UV data sampling the declining part of the synchrotron emission that, even during this extremely large outburst, had its maximum in the far-infrared. The X-ray spectral data from the XRT and BAT instruments are flat and due to inverse Compton emission. The remarkable SED observed implies that at the time of the Swift pointings 3C 454.3  was one of the brightest objects in the extragalactic sky with a γ-ray emission similar or brighter than that of 3C 279 when observed in a high state by EGRET. Time variability in the optical-UV flux is very different from that in the X-ray data: while the first component varied by about a factor two within a single exposure, but remained approximately constant between different observations, the inverse Compton component did not vary on short time-scales but changed by more than a factor of 3 between observations separated by a few days. This different dynamical behaviour illustrates the need to collect simultaneous multi-frequency data over a wide range of time-scales to fully constrain physical parameters in blazars.