ISO observational campaign of the BL Lac object PKS 2155-304

The BL Lacertae object PKS 2155-304 was observed by the Infrared Space Observatory in May and June 1996, during a multiwavelength campaign. These are the first observations in the mid- and far-infrared bands since IRAS. In the observing period, the source showed no detectable time variability at 4.0, 14.3, 60 and 90 micron. The spectrum from 2.8 to 100 micron is well fitted by a single power law with energy spectral index alpha = 0.4, intermediate between the flatter radio spectrum and the steeper simultaneous optical spectrum. The overall infrared to X-ray spectral energy distribution can be well explained by optically thin synchrotron emission, with negligible contributions from thermal sources. We also show that the host galaxy flux is negligible in this spectral range.