Hunting Distant BL Lacertae Objects with the Photometric Technique Using Swift and SARA

Abstract

BL Lacertae objects represent a large fraction (22%) of γ -ray sources in the Third Fermi Large Area Telescope catalog (3FGL). Nearly half of the BL Lac object population remains without a redshift because of their featureless optical spectra. We aim to increase the number of BL Lac objects with a redshift measurement by using the photometric technique. For this work, we have used six Swift-UVOT filters and SDSS g ′, r ′, i ′, and z ′ optical filters mounted on the 0.65 m SARA–CTIO located in Chile and the 1.0 m SARA–ORM in the Canary Islands. A sample of 45 sources was selected from the 3FGL catalog for which photometry was performed in 10 optical and UV filters to obtain redshift measurements. We found three sources with z  > 1.3, while reliable upper limits have been provided for 17 sources. The results presented here bring the total number of high- z ( z  > 1.3) BL Lac objects to 29.