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 \mu m. The spectrum from 2.8 to 100 \mu m 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 .