Classification of blazar candidates of uncertain type and estimation of Doppler factors

Context . The Fermi Gamma-ray Large Area Telescope Fourth Source Catalog Data Release 4 (4FGL-DR4) classifies 3935 γ-ray sources as blazars. They are divided into two types, based on the equivalent width (EW): flat spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs). However, 1625 sources lack optical spectroscopic data and are tentatively classified as blazar candidates of uncertain type (BCUs). Aims . This work aims to spectroscopically classify a sample of BCUs, determine the redshifts, and estimate the masses of the central supermassive black holes ( M BH ) and the Doppler factors (δ) of the jets, in order to better understand their physical properties and provide valuable classifications for Cherenkov observatories. Methods . We cross-matched BCUs from the 4FGL-DR4 with the Sloan Digital Sky Survey Data Release 16 (SDSS-DR16) to obtain optical spectra. Sources were classified based on the EW of emission lines. For spectra with broad emission lines, M BH was estimated using the virial method. For absorption line dominated spectra, M BH was derived from the stellar velocity dispersion of the host galaxy. The δ was estimated for FSRQs using an empirical relation between their γ-ray and broad-line region luminosities. Results . We obtained and analyzed optical spectra for the candidate counterparts of 86 BCUs. Based on the spectral features, we confirmed 35 sources as FSRQs, 45 as BL Lacs, 3 as low-ionization narrow emission-line regions (LINERs), and 3 as regular elliptical galaxies, and we secured redshifts for 54 objects. We corrected the erroneous SDSS redshift for the counterparts of 4FGL J1047.9+0055 from 0.252 to 0.642. The black hole masses in log[ M BH / M ⊙ )]range from 7.07 to 9.22. The derived δ for 35 FSRQs spans from 3.62 to 35.43, with a mean value ⟨δ⟩ of 13.41. The distribution of our classified sources in the γ-ray luminosity–photon index plane is consistent with that of the known blazars. Conclusions . Our spectroscopic analysis provides reliable classifications and key physical parameters for a large sample of BCUs. This work offers a crucial dataset for future statistical studies of blazars, their jet physics, and the connection between accretion processes and jet power.