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

TL;DR

This study developed a photometric method using Swift and SARA telescopes to identify high-redshift BL Lac objects, successfully discovering two new sources at z~1.8, enhancing the small existing sample and aiding extragalactic background light research.

Contribution

Introduced a novel photometric drop-out technique combining Swift and SARA data to determine redshifts of BL Lacs lacking spectral lines, discovering two high-z sources.

Findings

Identified two new high-redshift BL Lacs at z~1.8.

Added to the small sample of known high-z BL Lacs.

High-z BL Lacs may not fit existing blazar models.

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 $z=1.83^{+0.10}_{-0.13}$ and $z=1.73^{+0.11}_{-0.19}$ , 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.