2018ApJ...859...80K

BL Lacertae (BL Lac) objects are prominent members of the third Fermi Large Area Telescope catalog of γ-ray sources. Half of the members of the BL Lac population (∼300) lack redshift measurements, which is due to the absence of lines in their optical spectra, thereby making it difficult to utilize spectroscopic methods. Our photometric dropout technique can be used to establish the redshift for a fraction of these sources. This work employed six filters mounted on the Swift-UVOT and four optical filters on two telescopes, the 0.65 m SARA-CTIO in Chile and 1.0 m SARA-ORM in the Canary Islands, Spain. A sample of 15 sources was extracted from the Swift archival data for which six filter UVOT observations were conducted. By complementing the Swift observations with the SARA ones, we were able to discover two high-redshift sources: 3FGL J1155.4-3417 and 3FGL J1156.7-2250 at z=1.83_–0.13^+0.10 and z=1.73_–0.19^+0.11, respectively, resulting from the dropouts in the power-law template fits to these data. The discoveries add to the important (26 total) sample of high-redshift BL Lacs. While the sample of high-z BL Lacs is still rather small, these objects do not seem to fit well within known schemes of the blazar population and represent the best probes of the extragalactic background light.