New High-z BL Lacs Using the Photometric Method with Swift and SARA

A. Kaur, A. Rau, M. Ajello, A. Domínguez, V. S. Paliya, J. Greiner, D. H. Hartmann, P. Schady
2018 Astrophysical Journal  
BL Lacertae (BL Lac) objects are the prominent members of the third Fermi Large Area Telescope catalog of γ-ray sources. Half of the BL Lac population (∼ 300) lack redshift measurements, which is due to the absence of lines in their optical spectrum, thereby making it difficult to utilize spectroscopic methods. Our photometric drop-out technique can be used to establish the redshift for a fraction of these sources. This work employed 6 filters mounted on the Swift-UVOT and 4 optical filters on
more » ... wo 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 6 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^+0.10_-0.13 and z=1.73^+0.11_-0.19 , respectively, resulting from the dropouts in the powerlaw 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.
doi:10.3847/1538-4357/aabdec fatcat:dsixbze26bczxprypn2hlbdi64