Abstract
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 uvw2,uvm2,uvw1,u,b,v,g′,r′,i′,andz′ 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.
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