BL Lacertae identifications in a ROSAT-selected sample of Fermi unidentified objects

The optical spectroscopic followup of 27 sources belonging to a sample of 30 high-energy objects selected by positionally cross correlating the first Fermi/LAT Catalog and the ROSAT All-Sky Survey Bright Source Catalog is presented here. It has been found or confirmed that 25 of them are BL Lacertae objects (BL Lacs), while the remaining two are Galactic cataclysmic variables (CVs). This strongly suggests that the sources in the first group are responsible for the GeV emission detected with Fermi, while the two CVs most likely represent spurious associations. We thus find an 80% a posteriori probability that the sources selected by matching GeV and X-ray catalogs belong to the BL Lac class. We also show suggestions that the BL Lacs selected with this approach are probably high-synchrotron-peaked sources and in turn good candidates for the detection of ultra-high-energy (TeV) photons from them.

Based on observations collected at the following telescopes: Telescopio Nazionale Galileo of the Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias (Canary Islands, Spain); New Technology Telescope of the European Southern Observatory (ESO) in Cerro La Silla (Chile), under program 89.A-0148(A); Unit 2 "Kueyen" of the Very Large Telescope of the ESO in Cerro Paranal (Chile), under program 71.D-0176(A); 1.5-m telescope of the Cerro Tololo Interamerican Observatory (Chile).Tables 1-3 and Figs. 2-6 are available in electronic form at http://www.aanda.org