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

M. Rajagopal, A. Kaur, M. Ajello, A. Rau, A. Domínguez, B. Cenko, J. Greiner, D. H. Hartmann

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8 Scopus citations


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.

Original languageEnglish (US)
Article number18
JournalAstrophysical Journal
Issue number1
StatePublished - Jul 20 2020

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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