Extreme blazars

Abstract

TeV emission can be a common characteristic of low power blazars. This is in line with the sequence of blazars relating the observed bolometric luminosity with their overall spectral energy distribution. Detecting new TeV blazars, possibly at different redshifts, is important for studying the particle acceleration process operating in the jet, and it is even more important for studying the far infrared-optical background. Recent studies of low power BL Lacs suggest that the particle acceleration mechanism may be a two-step process, analogous to what is invoked to explain the spectra of gamma-ray bursts. I will briefly discuss these findings, together with a simple scheme for selecting good TeV candidates and I will briefly comment on the very fast TeV and X-ray variability observed in BL Lacs.