New high-z BL Lacs using the photometric method with Swift and SARA

Abstract

BL Lacertae (BL Lac) objects are the prominent members of the third {\it Fermi} Large Area Telescope catalog of $\gamma$-ray sources. Half of the BL Lac population ($\sim$ 300) lack redshift measurements, which is due to the absence of lines in their optical spectrum, thereby making it difficult to utilize spectroscopic methods. Our photometric drop-out technique can be used to establish the redshift for a fraction of these sources. This work employed 6 filters mounted on the $Swift$-UVOT and 4 optical filters on two telescopes, the 0.65 m SARA-CTIO in Chile and 1.0 m SARA-ORM in the Canary Islands, Spain. A sample of 15 sources was extracted from the $Swift$ archival data for which 6 filter UVOT observations were conducted. By complementing the {\it Swift} observations with the SARA ones, we were able to discover two high redshift sources: 3FGL J1155.4-3417 and 3FGL J1156.7-2250 at $z=1.83_{-0.13}^{+0.10}$ and $z=1.73_{-0.19}^{+0.11}$ , respectively, resulting from the dropouts in the powerlaw template fits to these data. The discoveries add to the important (26 total) sample of high-redshift BL Lacs. While the sample of high-z BL Lacs is still rather small, these objects do not seem to fit well within known schemes of the blazar population and represent the best probes of the extragalactic background light.

10 pages, 6 figures (accepted by ApJ)

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• Optical spectroscopy of Blazars for the Cherenkov Telescope Array
• Multiwavelength Study of High-Redshift Blazars
• Detection of a gamma-ray flare from the high-redshift blazar DA 193
• Optical spectral characterization of the gamma-ray blazars S4 0954+65, TXS 1515-273 and RX J0812.0+0237
• Predicting the redshift of gamma-ray loud AGNs using supervised machine learning
• Hunting distant BL Lacs with the photometric technique using Swift and SARA
• Using Neural Networks to Differentiate Newly Discovered BL Lacs and FSRQs among the 4FGL Unassociated Sources Employing Gamma-ray, X-ray, UV/Optical and IR Data
• Redshift prediction of Fermi-LAT gamma-ray sources using CatBoost gradient boosting decision trees
• Identifying the 3FHL catalog: V. Results of the CTIO-COSMOS optical spectroscopy campaign 2019
• First detection of VHE gamma-ray emission from TXS~1515--273, study of its X-ray variability and spectral energy distribution
• Classifying blazar candidates from the 3FGL unassociated catalog into BL Lacs and FSRQs using Swift and WISE data