Discovering High- z BL Lacs Using Swift/UVOT and SARA Observations with the Dropout Technique

Abstract Measuring spectroscopic redshifts for BL Lacertae (BL Lac) objects, a class of blazars, is challenging because their optical spectrum lacks, or has weak, emission lines (equivalent width ≤5 Å). In this situation, alternative techniques are necessary for the estimation of distances to these sources. In this paper, we estimate the redshift by the photometric dropout technique for a sample of 64 blazars (59 BL Lacs and 5 blazar candidates of uncertain type). Two telescopes are utilized to observe the sample. The Ultraviolet/Optical Telescope on board Swift observes sources in uvw 2, uvm 2, uvw 1, u , b , and v filters, while the ground-based SARA telescopes at Roque de los Muchachos, La Palma, Spain and Cerro Tololo, Chile observed sources in g ′ , r , ′ i ′ , z ′ filters. We fit the photometric data with the LePHARE package and report four new high- z ( z > 1.3) BL Lacs at 2.0 3 − 0.05 + 0.07 , 1.8 4 − 0.03 + 0.10 , 2.0 4 − 0.14 + 0.16 , and 2.9 3 − 0.04 + 0.01 as well as upper limits for 50 sources. This work increased the number of high- z BL Lacs found by this method up to 23. The high- z sources are discussed in the context of the cosmic gamma-ray horizon, blazar sequence, Fermi blazar divide, and masquerading BL Lacs.