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

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ISO observations of the BL Lacertae object PKS 2155-304 *

E. Bertone 1,2, G. Tagliaferri 1, G. Ghisellini 1, A. Treves 3, P. Barr 4, A. Celotti 5, M. Chiaberge 5 and L. Maraschi 6

1 Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (LC), Italy
2 Dipartimento di Fisica, Università degli Studi di Milano, via Celoria 16, 20133 Milano, Italy
3 Università dell'Insubria, via Lucini 3, 22100 Como, Italy
4 Astrophysics Division, ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
5 SISSA, via Beirut 2-4, 34014 Trieste, Italy
6 Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy

Received 18 January 2000 / Accepted 27 January 2000


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 µm. The spectrum from 2.8 to 100 µm is well fitted by a single power law with energy spectral index [FORMULA], 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.

Key words: galaxies: active – galaxies: BL Lacertae objects: individual: PKS 2155-304 – galaxies: photometry – infrared: general

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA.

Send offprint requests to: bertone@merate.mi.astro.it

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© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000