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High energy observations of extreme BL Lac objects, such as 1ES 0229+200 or1ES 0347-121, recently focused interest both for blazar and jet physics and forthe implication on the extragalactic background light and intergalacticmagnetic field estimate. However, the number of these extreme highly peaked BLLac objects (EHBL) is still rather small. Aiming at increase their number, weselected a group of EHBL candidates starting from the BL Lac sample of Plotkinet al. (2011), considering those undetected (or only barely detected) by theLarge Area Telescope onboard Fermi and characterized by a high X-ray vs. radioflux ratio. We assembled the multi-wavelength spectral energy distribution ofthe resulting 9 sources, profiting of publicly available archival observationsperformed by the Swift, Galex and Fermi satellites, confirming their nature.Through a simple one-zone synchrotron self-Compton model we estimate theexpected VHE flux, finding that in the majority of cases it is within the reachof present generation of Cherenkov arrays or of the forthcoming CherenkovTelescope Array (CTA).

An emerging population of BL Lacs with extreme properties: towards a class of EBL and cosmic magnetic field probes?