Optical spectroscopic observations of $\gamma$-ray blazar candidates VI. Further observations from TNG, WHT, OAN, SOAR and Magellan telescopes

TL;DR

This study reports optical spectroscopic observations of 30 gamma-ray blazar candidates, confirming their nature and demonstrating the effectiveness of multifrequency selection methods in identifying blazars among Fermi sources.

Contribution

It presents new optical spectra for 30 candidates, confirming 21 as BL Lac objects and validating the selection approach for identifying gamma-ray blazars.

Findings

21 out of 30 sources are BL Lac objects

Remaining sources are flat spectrum radio quasars

Multifrequency selection effectively identifies gamma-ray blazars

Abstract

Blazars, one of the most extreme class of active galaxies, constitute so far the largest known population of $\gamma$-ray sources and their number is continuously growing in the Fermi catalogs. However in the latest release of the Fermi catalog there is still a large fraction of sources that are classified as blazar candidate of uncertain type (BCUs) for which optical spectroscopic observations are necessary to confirm their nature and their associations. In addition about 1/3 of the $\gamma$-ray point sources listed in the Third Fermi-LAT Source Catalog (3FGL) are still unassociated and lacking an assigned lower energy counterpart. Since 2012 we have been carrying out an optical spectroscopic campaign to observe blazar candidates to confirm their nature. In this paper, the sixth of the series, we present optical spectroscopic observations for 30 $\gamma$-ray blazar candidates from different observing programs we carried out with the TNG, WHT, OAN, SOAR and Magellan telescopes. We found that 21 out of 30 sources investigated are BL Lac objects while the remaining targets are classified as flat spectrum radio quasars showing the typical broad emission lines of normal quasi stellar objects. We conclude that our selection of $\gamma$-ray blazars candidates based on their multifrequency properties continues to be a successful way to discover potential low-energy counterparts of the FermiUnidentified Gamma-ray Sources (UGSs) and to confirm the nature of BCUs.