2010MNRAS.408..422S

There are a total of 1451 gamma-ray emitting objects in the Fermi Large Area Telescope First Source Catalogue. The point source location accuracy of typically a few arcmin has allowed the counterparts for many of these sources to be found at other wavelengths, but even so there are 630 which are described as having no plausible counterpart at 80 per cent confidence. In order to help identify the unknown objects, we have cross-correlated the positions of these sources with the ROSAT All Sky Survey Bright Source Catalogue. In this way, for Fermi sources which have a possible counterpart in soft X-rays, we can use the much smaller ROSAT error box to search for identifications. We find a strong correlation between the two samples and calculate that there are about 60 sources with a ROSAT counterpart. Using the ROSAT error boxes we provide tentative associations for half of them, demonstrate that the majority of these are either blazars or blazar candidates, and give evidence that most belong to the BL Lac class. Given that they are X-ray selected and most are high synchrotron peaked objects, which indicates the presence of high-energy electrons, these sources are also good candidates for TeV emission, and therefore good probes of the extragalactic background light.