New High-z Fermi BL Lacs with the Photometric Dropout Technique

TL;DR

This paper introduces a photometric dropout technique using multi-band filters to determine redshifts of BL Lac objects, overcoming spectroscopic challenges, and reports new redshift measurements for 40 Fermi-detected BL Lacs.

Contribution

The study applies a novel photometric dropout method with broad-band filters to measure redshifts of BL Lacs, providing new redshift data where spectroscopy is difficult.

Findings

Redshifts determined for 5 BL Lacs, including a source at z=2.16.

Reliable upper limits established for 20 sources.

Results support the consistency of high-energy photon data with the gamma-ray horizon.

Abstract

Determining redshifts for BL Lacertae (BL Lac) objects using the traditional spectroscopic method is challenging due to the absence of strong emission lines in their optical spectra. We employ the photometric dropout technique to determine redshifts for this class of blazars using the combined 13 broad-band filters from Swift-UVOT and the multi-channel imager GROND at the MPG 2.2 m telescope at ESO's La Silla Observatory. The wavelength range covered by these 13 filters extends from far ultraviolet to the near-Infrared. We report results on 40 new Fermi detected BL Lacs with the photometric redshifts determinations for 5 sources, with 3FGL J1918.2-4110 being the most distant in our sample at z=2.16. Reliable upper limits are provided for 20 sources in this sample. Using the highest energy photons for these Fermi-LAT sources, we evaluate the consistency with the Gamma-ray horizon due to the extragalactic background light.