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Astron. Astrophys. 356, 1-10 (2000)

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1. Introduction

BL Lacertae objects are characterized by an intense and variable non-thermal continuum, that extends from the radio to the gamma-ray band. This is commonly attributed to synchrotron and inverse Compton radiation from a relativistic jet pointing toward the observer (see Ulrich et al. 1997 for a review). In a [FORMULA] representation, their overall spectrum has two broad peaks, one at low energies (IR-X) due to synchrotron radiation and one at higher energies (X-[FORMULA]), plausibly due to inverse Compton scattering.

PKS 2155-304 is one of the brightest BL Lacs from the optical to the X-ray band with the synchrotron peak in the UV- soft X-ray range, corresponding to the definition of High frequency peak BL Lac objects (HBL) (Padovani & Giommi 1995), which have the synchrotron peak at the highest frequencies, low luminosity and a small ratio between the [FORMULA]-ray and the synchrotron peak luminosities. The gamma-ray spectrum is flat ([FORMULA] 1 in the 0.1-10 GeV energy range), indicating that the Compton peak is beyond [FORMULA] GeV. Recently it has been detected in the TeV band (Chadwick et al. 1999). Due to these characteristics, PKS 2155-304 has been the target of numerous multiwavelength campaigns (e.g. Edelson et al. 1995for November 1991, Urry et al. 1997for May 1994). The study of the simultaneous behavior of the source at different frequencies is important in order to understand the emission mechanisms and to constrain the physical properties of the emitting region.

In 1996 May-June, an intense multiwavelength monitoring was carried out involving optical telescopes, UV, X-ray and [FORMULA]-ray satellites. Thanks to the Infrared Space Observatory (ISO), for the first time we had infrared simultaneous observations. These are the first observations of this object in the mid- and far- infrared since IRAS. PKS 2155-304 was detected by IRAS in 1983 at 12, 25, 60 microns with a flux of about 100 mJy in all three bands (Impey & Neugebauer 1988). In this object the IR emission is at frequencies lower than the synchrotron peak, and the spectral shape in this band can reveal if there are relevant thermal contributions (e.g. by the host galaxy or by a dusty torus around the nucleus) or if the emission can be entirely attributed to synchrotron radiation.

Here we present the ISO observations of PKS 2155-304, carried out during the campaign in 1996 May-June, covering a wavelength range from 2.8 to 200 µm. This is complemented by some simultaneous BVR observations from the Dutch 0.9 m ESO telescope. Results from ISO observations of 1996 November and 1997 May are also reported.

The paper is organized as follows: a brief description of the ISO instruments and of the observations are given in Sect. 2 and the results are reported in Sect. 3. In Sect. 4 we present the optical data and in Sect. 5 we compare our results with the theoretical models. PKS 2155-304 is a weak IR source for ISO. Therefore considerable care was taken in data reduction and background subtraction. Details are given in Appendix A.

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© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000
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