Energy distribution and variability of BL Lac objects: Recent observations and a theoretical scheme

Abstract

Spectral properties of X-ray selected and radio-selected BL Lacs are discussed. The results provide a direct indication that the X-ray emission is more isotropic than the radio emission. Recent results of multifrequency observations of the brightest X-ray selected BL Lac object, PKS 2155-304, are reviewed, showing how the fast X-ray variability is smeared out at low frequencies. A theoretical jet model is presented which can quantitatively account for the different spectral properties of X-ray selected and radio selected BL Lacs in terms of different line-of-sight angles to the jet.