TeV BL Lac objects at the dawn of the Fermi era

Abstract

We reconsider the emission properties of the BL Lac objects emitting in the high-energy gamma-ray band exploiting the new information in the MeV-GeV band obtained by the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space Telescope in its first three months of operation. To this aim we construct the spectral energy distribution of all the BL Lacs revealed by LAT and of the known TeV BL Lacs not detected by LAT, also including data from the Swift satellite, and model them with a simple one-zone leptonic model. The analysis shows that the BL Lacs detected by LAT (being or not already detected in the TeV band) share similar physical parameters. While some of the TeV BL Lacs not revealed by LAT have spectral energy distributions and physical parameters very similar to the LAT BL Lacs, a group of objects displays peculiar properties (larger electron energies and smaller magnetic fields) suggesting different physical conditions in the emission region. Finally, we discuss possible criteria to effectively select good new candidates for the Cherenkov telescopes among the LAT sources, presenting a list of predicted fluxes in the very high-energy band calculated including the effect of the absorption by the extragalactic background light.