Optical spectroscopic observations of low-energy counterparts of Fermi-LAT gamma-ray sources

TL;DR

This study reports on a decade-long optical spectroscopic campaign that classified and discovered numerous blazar counterparts to Fermi-LAT gamma-ray sources, significantly improving source identification and understanding of gamma-ray populations.

Contribution

The paper presents a comprehensive optical spectroscopic campaign that classified 394 new blazar counterparts and identified potential counterparts for about 10% of unassociated gamma-ray sources in Fermi-LAT catalogs.

Findings

Classified 394 targets as blazars.

Discovered blazar-like counterparts for ~10% of unassociated sources.

Contributed to about 350 classifications in the 4FGL catalog.

Abstract

A significant fraction of all $\gamma$-ray sources detected by the Large Area Telescope aboard the \fer\ satellite is still lacking a low-energy counterpart. In addition, there is still a large population of $\gamma$-ray sources with associated low-energy counterparts that lack firm classifications. In the last 10 years we have undertaken an optical spectroscopic campaign to address the problem of unassociated/unidentified $\gamma$-ray sources (UGSs), mainly devoted to observing blazars and blazar candidates because they are the largest population of $\gamma$-ray sources associated to date. Here we describe the overall impact of our optical spectroscopic campaign on sources associated in \fer-LAT catalogs, coupled with objects found in the literature. In the literature search, we kept track of efforts by different teams that presented optical spectra of counterparts or potential counterparts of \fer-LAT catalog sources. Our summary includes an analysis of an additional 30 newly-collected optical spectra of counterparts or potential counterparts of \fer-LAT sources of previously unknown nature.New spectra were acquired at the Blanco 4-m and OAN-SPM 2.1-m telescopes, and those available in the Sloan Digital Sky Survey (data release 15) archive. All new sources with optical spectra analyzed here are classified as blazars. Thanks to our campaign, we altogether discovered and classified 394 targets with an additional 123 objects collected from a literature search. We began our optical spectroscopic campaign between the release of the second and third \fer-LAT source catalogs (2FGL and 3FGL, respectively), and classified about 25\% of the sources with uncertain nature and discovered a blazar-like potential counterpart for $\sim$10\% of UGSs listed therein. In the 4FGL catalog, about 350 \fer-LAT sources are classified to date thanks to our campaign. [incomplete abstract]