A new look at the extragalactic very high energy sky: Searching for TeV-emitting candidates among the X-ray-bright, non-Fermi-detected blazar population

We present the results of a multiwavelength study of blazars selected from the fifth ROMABZCAT catalog. We selected from this sample a subsample of 2435 objects that have at least one counterpart in one of the three main archival X-ray catalogs, namely the fourth release of the XMM-Newton Survey Science Catalogue, the second release of the Chandra Source Catalog, and the second Swift X-ray Point Source catalog of detections by Swift-XRT, or in the recently released eROSITA-DE Data Release 1 catalog. We first searched for different multiwavelength trends between sources with a γ-ray counterpart in the Fermi-LAT 14-year Source Catalog (4FGL–DR4) and sources lacking one. We find that the non-4FGL sources are on average fainter both in the X-rays and in the radio with respect to the 4FGL-detected ones, but that the two samples have similar X-ray-to-radio flux ratios and synchrotron peak frequencies. We then focused on the 1007 non-γ-ray detected population to determine whether or not there is a sample of X-ray sources that could be TeV emitters. We find that a large number of sources – mostly BL Lacs or BL Lacs with host-galaxy contribution to the spectral energy distribution – have a large synchrotron peak frequency and a large X-ray-to-radio flux ratio, two properties that characterize the vast majority of known TeV emitters. With respect to these known TeV emitters, our targets have X-ray fluxes that are about one order of magnitude fainter. We then computed the 0.2–12 keV and 20 GeV–300 TeV fluxes for the known 5BZCAT TeV emitters, and find a direct correlation between X-ray and TeV fluxes in the BL Lacs population. We used this trend to estimate the VHE flux of our targets, and find a promising sample of sources for follow-up observations with current or future, more sensitive Cherenkov telescopes; first and foremost the Cherenkov Telescope Array Observatory.