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 uvw 2 , uvm 2 , uvw 1 , u , b , v , g ′ , r ′ , i ′ , and 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 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.