Ja n 20 07 X – ray / UV / Optical follow - up of the blazar PKS 2155 – 304 after the giant TeV flares of July 2006

We present all the publicly available data, from optical/UV wavelengths (UVOT) to X–rays (XRT, BAT), obtained from Swift observations of the blazar PKS 2155–304, performed in response to the rapid alert sent out after the strong TeV activity (up to 17 Crab flux level at E > 200 GeV) at the end of July 2006. The X-ray flux increased by a factor of 5 in the 0.3−10 keV energy band and by a factor of 1.5 at optical/UV wavelengths, with roughly one day of delay. The comparison of the spectral energy distribution built with data quasi-simultaneous to the TeV detections shows an increase of the overall normalization with respect to archival data, but only a small shift of the frequency of the synchrotron peak that remains consistent with the values reported in past observations when the TeV activity was much weaker. Subject headings: BL Lacertae objects: general – BL Lacertae objects: individual: PKS 2155-304 INAF/IASF-Bologna, Via Gobetti 101, 40129 Bologna (Italy) INAF/Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano (Italy) Dipartimento di Scienze, Università dell’Insubria, Via Vallegio 11, 22100 Como (Italy) George Mason University, Department of Physics and Astronomy, School of Computational Sciences, Mail Stop 3F3, 4400 University Drive, Fairfax, VA 22030, USA INAF/Osservatorio Astronomico di Torino, Via Osservatorio 20, 10025 Pino Torinese (Italy) INAF/Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34131 Trieste (Italy) Osservatorio Astronomico, Università di Perugia, Via B. Bonfigli, 06126 Perugia (Italy) NASA/Goddard Space Flight Center, Code 661, Greenbelt, MD 20771, USA ASI Science Data Centre, Via G. Galilei, 00044 Frascati, (Italy) Email: [email protected]