Discovering High-$z$ BL Lacs Using Swift/UVOT and SARA Observations with the Dropout Technique

Abstract

Measuring spectroscopic redshifts for BL Lacertae (BL Lac) objects, a class of blazar, is challenging because their optical spectrum lacks, or has weak, emission lines ( equivalent width $⩽5\tilde{A}$). 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 five blazar candidates of uncertain type). Two telescopes are utilized to observe the sample. The Ultraviolet/Optical Telescope (UVOT) on board {\it Swift} ({\it Swift}/UVOT) observes sources in $uvw2,uvm2,uvw1,u,b,v$ filters, while the ground-based telescopes SARA-CT/RM 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 $2.03_{-0.05}^{+0.07}$, $1.84_{-0.03}^{+0.10}$, $2.04_{-0.14}^{+0.16}$, and $2.93_{-0.04}^{+0.01}$ 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.

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