J/A+A/707/A347 Blazar candidates of uncertain type (Chen+, 2026)
Classification of blazar candidates of uncertain type and estimation of
Doppler factors.
Chen G., Yang W., Ye X., Yuan Y., Feng J., Fan J.
<Astron. Astrophys. 707, A347 (2026)>
=2026A&A...707A.347C 2026A&A...707A.347C (SIMBAD/NED BibCode)
ADC_Keywords: QSOs ; Radio sources ; Spectra, optical
Keywords: galaxies: active - BL Lacertae objects: general - galaxies: jets
Abstract:
The Fermi Gamma-ray Large Area Telescope Fourth Source Catalog Data
Release 4 (4FGL-DR4) classifies 3935 gamma-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).
This work aims to spectroscopically classify a sample of BCUs,
determine the redshifts, and estimate the masses of the central
supermassive black holes (MBH) and the Doppler factors (delta) of the
jets, in order to better understand their physical properties and
provide valuable classifications for Cherenkov observatories.
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, MBH was estimated using the virial method. For
absorption line dominated spectra, MBH was derived from the stellar
velocity dispersion of the host galaxy. The delta was estimated for
FSRQs using an empirical relation between their gamma-ray and
broad-line region luminosities.
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[MBH/M☉)]range from 7.07 to
9.22. The derived delta for 35 FSRQs spans from 3.62 to 35.43, with a
mean value (delta) of 13.41. The distribution of our classified
sources in the gamma-ray luminosity-photon index plane is consistent
with that of the known blazars.
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.
Description:
We obtained optical spectra for the candidate counterparts of 86 BCUs
by cross-matching with SDSS-DR16. Based on detailed spectral analysis,
we classified 35 sources as FSRQs, 45 as BL Lacs, 3 as LINERs, and 3
as regular elliptical galaxies, and secured redshifts for 54 objects.
Notably, we identified and corrected an erroneous redshift in SDSS for
4FGL J1047.9+0055 (from z=0.252 to z=0.642). The distribution of
our classified sources in the gamma-ray luminosity versus photon index
plane shows excellent consistency with firmly identified blazars,
validating our classification scheme.
Furthermore, we estimated the MBH for the sample. For emission line
objects, MBH was derived using virial methods based on the broad
emission line properties, while for absorption-line dominated spectra,
it was calculated from the stellar velocity dispersion of the host
galaxy. The black hole masses span a range of
log(MBH/M[sun)=7.07-9.22.
For the 35 FSRQs in our sample, we estimated delta using an empirical
relation that connects the gamma-ray and broad-line region
luminosities. The derived delta range from 3.62 to 35.43, with a mean
value of =13.41.
This study provides robust classifications, redshifts, and key
physical parameters (MBH and delta) for a significant sample of
previously uncertain gamma-ray sources. These results not only
substantially reduce the number of BCUs, but also provide a crucial
foundation for future statistical studies of blazar populations, jet
physics, and the connection between accretion processes and jet
launching mechanisms.
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 156 86 Cross-matched results of BCU samples
with SDSS spectra
table2.dat 114 237 Spectral line measurements of BCU samples
with SDSS spectra
table3.dat 125 46 Black hole mass estimates and derived parameters
for BCU samples
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See also:
IX/67 : Incremental Fermi LAT 4th source cat. (4FGL-DR3) (Fermi-LAT col., 2022)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 12 A12 --- 4FGL 4FGL name
14- 41 A28 --- Ctp Associated counterpart name of the
Fermi source
47- 62 A16 --- SDSS SDSS spectral identifier in
Plate-MJD-FiberID format
64- 71 F8.4 deg RAdeg Right Ascension of Fermi 4FGL source
centroid (J2000)
74- 81 F8.4 deg DEdeg Declination of Fermi 4FGL source centroid
(J2000)
83- 91 F9.5 deg RACdeg Right Ascension of Fermi associated
counterpart (J2000)
94-102 F9.5 deg DECdeg Declination of Fermi associated counterpart
(J2000)
104-112 F9.5 deg RASdeg Right Ascension of matched SDSS spectrum
(J2000)
115-123 F9.5 deg DESdeg Declination of matched SDSS spectrum
(J2000)
127-131 F5.3 deg SepFermiSDSS Angular separation between Fermi centroid
and SDSS spectrum
135-139 F5.3 deg ePosFermi 95% positional error radius of Fermi 4FGL
source centroid
144-148 F5.3 arcsec SepCtpSDSS Angular separation between associated
counterpart and SDSS spectrum, arcseconds
152-156 F5.3 --- ProbAssoc Association probability given in the 4FGL
catalog
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Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 12 A12 --- 4FGL 4FGL name
14- 30 A17 --- SDSS SDSS spectral identifier in
Plate-MJD-FiberID format
32- 36 F5.2 --- SNR Signal-to-noise ratio of the spectrum
39- 44 F6.4 --- zSDSS ? Redshift in SDSS
46 A1 --- Class Classification based on optical spectra (1)
51- 57 F7.2 0.1nm lambdaObs ? Observed wavelength of the emission or
absorption line (Angstroems)
59 A1 --- LineType [AE] Line type: E for emission lines,
A for absorption lines
61- 85 A25 --- LineID Line identification with rest-frame
wavelength (2)
88- 93 F6.4 --- zline ? Redshift derived from the line
95-100 F6.4 --- e_zline ? Uncertainty in z_line
102-107 F6.2 0.1nm EW ? Equivalent width of the line (Angstroms)
110-114 F5.2 0.1nm e_EW ? Uncertainty in EW
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Note (1): Classification as follows:
F = FSRQ
B = BL Lac
L = LINER
G = regular elliptical galaxy
Note (2): H-alpha* (H-beta*) denotes a narrow H-alpha (H-beta) emission line.
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Byte-by-byte Description of file: table3.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 12 A12 --- 4FGL 4FGL name
15- 20 F6.4 --- zSDSS Redshift in SDSS
21- 28 A8 --- LineID Emission line used for black hole mass
estimation
30- 34 F5.2 10-7W Lcont ? Continuum luminosity (erg/s) (1)
36- 39 F4.2 10-7W e_Lcont ? Continuum luminosity error (erg/s) (1)
42- 49 F8.2 km/s FWHM Full width at half maximum of the
emission line
51- 57 F7.2 km/s e_FWHM Full width at half maximum of the
emission line error
60- 66 F7.2 10-20W/m2 Flux Line flux (10-17erg/s/cm2^)
68- 73 F6.2 10-20W/m2 e_Flux Line flux error (10-17erg/s/cm2^)
75- 79 F5.2 [10-7W] logLline Logarithmic line luminosity (erg/s)
82- 85 F4.2 [10-7W] e_logLline Logarithmic line luminosity error (erg/s)
87- 90 F4.2 [Msun] logMBH ? Logarithmic black hole mass
93- 96 F4.2 [Msun] e_logMBH ? Logarithmic black hole mass error
98-102 F5.2 [10-7W] logLBLR ? Logarithmic broad-line region
luminosity (erg/s)
104-107 F4.2 [10-7W] e_logLBLR ? Logarithmic broad-line region
luminosity error (erg/s)
109-113 F5.2 [10-7W] logLgamma ? Logarithmic gamma-ray luminosity,
log L_gamma(0.1-100 GeV) (erg/s)
115-118 F4.2 [10-7W] e_logLgamma ? Logarithmic gamma-ray luminosity error
(erg/s)
121-125 F5.2 --- Doppler ? Doppler factor estimated in this work
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Note (1): Typical wavelengths: 5100A for Hbeta, 3000A for MgII, 1350A for CIV.
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Acknowledgements:
Guohai Chen, cgh(at)e.gzhu.edu.cn
(End) Patricia Vannier [CDS] 17-Feb-2026