Revealing High-z Fermi-LAT BL Lacs Using Swift and SARA Data with Photometric Analysis

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 uvm2, uvw1, u, b, v, g ', r ', i ', z ' 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 1.74-0.08+0.051.74-0.08+0.05, 1.88-0.03+0.071.88-0.03+0.07, and 2.10-0.04+0.032.10-0.04+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 \emph{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.