Extreme TeV blazars and the intergalactic magnetic field

We study the four BL Lac objects (RGBJ0152+017, IES 0229+200, 1ES 0347―121 and PKS 0548-322) detected in the TeV band but not present in the first Fermi-LAT (1FGL) catalogue of the Fermi/Large Area Telescope (LAT). We analyse the 24 months of LAT data deriving γ-ray fluxes or upper limits that we use to assemble their spectral energy distributions (SED). We model the SEDs with a standard one-zone leptonic model, also including the contribution of the reprocessed radiation in the multi-GeV band, emitted by the pairs produced through the conversion of the primary TeV emission by interaction with the cosmic optical-infrared background. For simplicity, in the calculation of this component we adopt an analytical approach including some simplifying assumptions, in particular (i) the blazar high-energy emission is considered on average stable over times of the order of 10 7 yr and (ii) the observer is exactly on-axis. We compare the physical parameters derived by the emission model with those of other high-energy emitting BL Lacs, confirming that TeV BL Lacs with a rather small GeV flux are characterized by extremely low values of the magnetic field and large values of the electron energies. The comparison between the flux in the GeV band and that expected from the reprocessed TeV emission allows us to confirm and strengthen the lower limit of B ≳ 10 ―15 G for the intergalactic magnetic field using a theoretically motivated spectrum for the primary high-energy photons.