# Optical spectroscopic observations of gamma-ray blazar candidates VIII: the 2016-2017 follow up campaign carried out at SPM, NOT, KPNO and SOAR telescopes

## TL;DR

This study presents optical spectroscopic observations of gamma-ray blazar candidates, confirming their nature and estimating redshifts, thereby improving classification of unassociated gamma-ray sources from the Fermi catalog.

## Contribution

It provides new spectroscopic data confirming blazar types and redshifts for 36 gamma-ray source candidates, enhancing the understanding of their nature and distribution.

## Key findings

- Confirmed 23 BL Lac objects and 7 flat spectrum radio quasars.
- Provided redshift estimates for 19 sources, including a distant BL Lac at z>1.529.
- Most classified BCUs are BL Lac objects, highlighting their elusive nature.

## Abstract

The third Fermi source catalog lists 3033 gamma-ray sources above $4\sigma$ significance. More than 30% are classified as either unidentified/unassociated Gamma-ray sources (UGSs), with about 20% classified as Blazar candidates of uncertain types (BCUs). To confirm the blazar-like nature of candidate counterparts of UGSs and BCUs, we started in 2012 an optical spectroscopic follow up campaign. We report here the spectra of 36 targets with observations from the Observatorio Astron\'omico Nacional San Pedro M\'artir, the Southern Astrophysical Research Observatory, the Kitt Peak National Observatory and the Northern Optical Telescope, between 2016 and 2017. We confirm the BL Lac nature of 23 sources, and the flat spectrum radio quasar nature of other 7 ones. We also provide redshift estimates for 19 out of these 30 confirmations, with only one being a lower limit due to spectral features ascribable to intervening systems along the line of sight. As in previous analyses, the largest fraction of now-classified BCUs belong to the class of BL Lac objects, that appear to be the most elusive class of active galactic nuclei. One of the BL Lacs identified in this work, associated with 3FGL J2213.6-4755, lies at a redshift of $z>$1.529, making it one of the few distant gamma-ray BL Lac objects.