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

¹ Center for Astrophysics, Guangzhou University, Guangzhou 510006, China
² Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, Via F. Marzolo 8, 35131 Padova, Italy
³ Greater Bay Brand Center of the National Astronomical Data Center, Guangzhou 510006, China
⁴ Astronomy Science and Technology Research Laboratory of Department of Education of Guangdong Province, Guangzhou 510006, China

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 November 2025
Accepted: 4 February 2026

Abstract

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.