2022MNRAS.516.5702O

The classification of γ-ray-detected blazar candidates of uncertain type (BCU) is a relevant problem in extragalactic γ-ray astronomy. Here, we report the optical spectroscopic characterization, using two 3-4 m class telescopes, Telescopio Nazionale Galileo and Devasthal Optical Telescope, of 27 BCUs detected with the Fermi Large Area Telescope. Since the identification of emission lines is easier in broad-line blazars, which usually exhibit low frequency peaked (synchrotron peak frequency ≤10¹⁴ Hz) spectral energy distribution, we primarily target such BCUs. We found that 8 out of 27 sources exhibit broad emission lines in their optical spectra, 3 of them have redshifts >1 and the farthest one is at z = 2.55. The optical spectra of 2 of the 19 remaining objects are dominated by the absorption spectra of the host galaxy, and there is a tentative detection of the Lyman-α absorption feature in one source. The spectra of the remaining 16 objects, on the other hand, are found to be featureless.