SIMBAD references

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 uvw2, uvm2, uvw1, 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 ${g}^{{\prime} },\,r{,}^{{\prime} }\,{i}^{{\prime} },\,{z}^{{\prime} }$ 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.