Internal shocks and the blazar sequence - Low and intermediate power BL Lac objects

We consider internal shocks as the main dissipation mechanism responsible for the emission in blazars and show that it can satisfactorily account for the properties of all blazars. In particular, we extend previous work (Spada et al. 2001) on powerful objects, to intermediate (BL Lac) and low power sources (Mkn 421), in order to reproduce the whole of the blazar sequence. The model self-consistently treats the dynamics, spectral emission and its variability. The key parameters driving the phenomenological sequence are the jet power and the properties of the broad line region, which regulate the cooling efficiency of the emitting particles and thus the shape of the spectral energy distribution. By assuming that the remaining parameters are similar for all objects it has been possible to reproduce the full range of the observed spectral "states". A more detailed comparison of the variability properties shows (for Mkn 421) a good agreement in the X-ray band, while in the optical the simulated flux appears to be too variable. For BL Lac lags (similar to10 days) are predicted between the gamma-rays and the infrared emission.