Optical spectroscopic survey of a sample of Unidentified Fermi objects

TL;DR

This study uses optical spectroscopy to identify and determine redshifts of extragalactic gamma-ray sources detected by Fermi, confirming their nature as BL Lac objects and providing insights into their properties.

Contribution

First optical spectroscopic survey of Fermi-detected gamma-ray sources, confirming their BL Lac nature and establishing redshift measurements for multiple objects.

Findings

All counterparts are consistent with BL Lac objects.

Redshifts determined for 11 sources, with limits for 5.

Featureless spectra in 4 sources, setting redshift lower limits.

Abstract

We present optical spectroscopy secured at the 10m Gran Telescopio Canarias of the counterparts of 20 extragalactic gamma-ray sources detected by the Fermi satellite. The observations allow us to investigate the nature of these sources and to determine their redshift. We find that all optical counterparts have a spectrum that is consistent with a BL Lac object nature. We are able to determine the redshift for 11 objects and set spectroscopic redshift limits for five targets. Only for four sources the optical spectrum is found featureless. In the latter cases we can set lower limits on the redshift based on the assumption that they are hosted by a typical massive elliptical galaxy whose spectrum is diluted by the non thermal continuum. The observations allow us to unveil the nature of these gamma-ray sources and provide a sanity check of a tool to discover the counterparts of gamma-ray emitters/blazars based on their multiwavelength emission.