SDSS J154958.45+631021.2 , the SIMBAD biblio

BL Lacertae (BL Lac) objects are a subclass of blazar, distinguished by their featureless optical spectrum. The featureless spectrum presents a challenge in measuring the redshift of the BL Lacs. In this paper, we measure the redshift of BL Lacs using the photometric dropout technique. The space-based telescope Swift and the ground-based SARA telescopes are employed to provide magnitudes in the ${uvw}2,{uvm}2,{uvw}1,u,b,v,g^{\prime} ,r^{\prime} ,i^{\prime} ,{\rm{and}}\,z^{\prime} $ filters. We observe 60 BL Lacs and report reliable redshift upper limits for 41 of them. We discover three new high-z BL Lacs (z > 1.3) at redshifts of ${1.74}_{-0.08}^{+0.05}$, ${1.88}_{-0.03}^{+0.07}$, and ${2.10}_{-0.04}^{+0.03}$, bringing the number of high-z BL Lacs found by this method up to 19. Discussions are made on the implications for the blazar sequence, the Fermi blazar divide, and the gamma-ray horizon based on an analysis of the 4LAC catalog and all high-z BL Lacs found with the photo-z technique.