REM near-IR and optical multiband observations of PKS 2155-304 in 2005^*

¹ Universit degli Studi dell'Insubria, Dipartimento di Fisica e Matematica, via Valleggio 11, 22100 Como, Italy e-mail: alberto.dolcini@gmail.com
² Dipartimento di Fisica e Osservatorio Astronomico, Universit di Perugia, via. A. Pascoli, 06123 Perugia, Italy
³ INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁴ INAF - Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
⁵ Universit degli Studi di Milano-Bicocca, Dipartimento di Fisica, Piazza delle Scienze, 3, 20126 Milan, Italy
⁶ INAF/IASF Bologna, via Gobetti 101, 40129 Bologna, Italy
⁷ INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
⁸ Dunsink Observatory, Castleknock, Dublin 15, Ireland
⁹ Observatori Astronomic, Universitat de Valencia, Aptdo. Correos 22085, Valencia 46071, Spain
¹⁰ INAF - Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy

Received: 29 September 2006
Accepted: 26 March 2007

Abstract

Context.Spectral variability is the main tool for constraining emission models of BL Lac objects.

Aims.By means of systematic observations of the BL Lac prototype PKS 2155-304 in the infrared-optical band, we explore variability on scales of months, days, and hours.

Methods.We made our observations with the robotic 60 cm telescope REM located at La Silla, Chile, and VRIJHK filters were used.

Results.PKS 2155-304 was observed from May to December 2005. The wavelength interval explored, the total number of photometric points, and the short integration time render our photometry substantially superior to previous ones for this source. On the basis of the intensity and colour, we distinguish three different states of the source, each lasting months, which include all those described in the literature. In particular, we report the highest state ever detected in the H band. The source varied by a factor of 4 in this band, much more than in the V band (a factor ≈2). The source softened with increasing intensity, in contrast to the general pattern observed in the UV-X-ray bands. On five nights in November we had nearly continuous monitoring for 2–3 h. A variability episode on a time scale of 24 h is well-documented, and a much more rapid flare with τ = 1–2 h, is also apparent, but is supported by relatively few points.

Conclusions.The overall spectral energy distribution of PKS 2155-304 is commonly described by a synchrotron-self-Compton model. The optical infrared emission is in excess of the expectation of the model in its original formulation. This can be explained by a variation in the frequency of the synchrotron peak, which is not unprecedented in BL Lacs.