Spectroscopy of a Sample of Unidentified Gamma-ray Fermi Sources

The fourth-DR3 version (4FGL-DR3) of the Fermi/LAT catalogue of γ-ray sources contains ∼ 1000 objects at a galactic latitude |b| > 10^∘ which are not identified with an optical counterpart (UGS). We performed a systematic study of these sources, focusing on 190 objects that have a unique X-ray counterpart in the available Swift/XRT observations. Optical counterparts are then selected, and for 33 sources optical spectra were found in the literature. We found that 21 can be classified as BL Lac objects. Among these we were able to provide the redshift for 8 of them while for 2 others we established a lower limit to the redshift by detecting intervening absorption. The other 12 objects display optical spectra with prominent emission lines (0.036更多