The broad band energy distribution of UV-bright BL Lac objects

Abstract

Renewed attention was paid to the study of the continuous energy distribution of BL Lac objects, bearing on the emission mechanisms of the central source, since the launch of the International Ultraviolet Explorer (IUE). The far ultraviolet domain (1000 - 3000 Å) is in fact particularly relevant to the problem, because it does not suffer from contamination by star light of the host galaxy. Moreover, the reddening correction can in principle be determined from the rectification of the 2200 Å dip, provided that a suitable signal-to-noise ratio is achieved. 25 BL Lac objects have been observed with IUE so far by a number of different authors. For all objects, information on their ultraviolet emission has been gathered either from the literature or by direct analysis of archival data. The properties of the sample in the ultraviolet are discussed and compared with those deduced from observations at other wavelengths.