J/ApJ/999/244 X-ray catalog and classification of 4FGL-DR4 UGSs (Neumann+ 2026)
Classification of a New X-Ray Catalog of Likely Counterparts to 4FGL-DR4
Unassociated Gamma-Ray Sources Using a Neural Network.
Neumann K.D., Falcone A.D., Dikerby S., Deppe S., Ferrara E.C.,
Kennea J.A., Cenko B., Grove E.
<Astrophys. J. 999, 244 (2026)>
=2026ApJ...999..244N 2026ApJ...999..244N (SIMBAD/NED BibCode)
ADC_Keywords: Gamma rays; X-ray sources; Photometry, UBV; Active gal. nuclei;
BL Lac objects; Pulsars
Keywords: Catalogs - X-ray astronomy - Ultraviolet astronomy - Neural networks
Abstract:
Our survey of the fourth Fermi Large Area Telescope catalog (4FGL)
unassociated gamma-ray source regions using the X-Ray Telescope (XRT)
and Ultraviolet/Optical Telescope (UVOT) on board the Neil Gehrels
Swift Observatory (Swift) provides new XRT and UVOT source detections
and localizations to help identify potential low-energy counterparts
to unassociated Fermi gamma-ray sources. We present a catalog of 218
singlet and 70 multiplet Swift X-ray sources detected within the
positional uncertainty ellipses of 244 unassociated Fermi gamma-ray
sources from the 4FGL-DR4 catalog, 144 of which are not previously
cataloged by S. Kerby et al (2021ApJ...923...75K 2021ApJ...923...75K, Cat. J/ApJ/923/75).
For each X-ray source, we derive its X-ray flux and photon index, then
use simultaneous UVOT observations with optical survey data to
estimate its V-band magnitude. We use these parameters as inputs for a
multilayer perceptron neural network classifier (NNC) trained to
classify sources as blazars, pulsars, or ambiguous gamma-ray sources.
For the 213 singlet sources with X-ray and optical data, we classify
173 as likely blazars (Pbzr>0.99) and six as likely pulsars
(Pbzr<0.01), with 34 sources yielding ambiguous results. Including
70 multiplet X-ray sources, we increase the number of Pbzr>0.99
sources to 227 and Pbzr<0.01 sources to 16. For the subset of these
classifications that have been previously studied, a large majority
agree with prior classifications, supporting the validity of using
this NNC to classify the unknown and newly detected gamma-ray sources.
Description:
Our survey of data release (DR4, Ballet et al., 2023,
2023arXiv230712546B 2023arXiv230712546B) the fourth Fermi Large Area Telescope catalog
(4FGL, Abdollahi et al., 2020ApJS..247...33A 2020ApJS..247...33A) unassociated gamma-ray
source regions (UGSs) using the X-Ray Telescope (XRT) and
Ultraviolet/Optical Telescope (UVOT) on board the Neil Gehrels Swift
Observatory (Swift) provides new XRT and UVOT source detections and
localizations to help identify potential low-energy counterparts to
unassociated Fermi gamma-ray sources. The unassociated sources of the
4FGL-DR4 sample are those lacking confident astrophysical
classification or lower-energy counterparts in the 14-year source
catalog. Swift surveys all Fermi unassociated sources not labeled as
'extended' or 'confused' with interstellar cloud complexes to detect
low-energy counterparts as part of our ongoing observing campaign.
These provide high-resolution X-ray and UV/optical observations of
UGSs to identify likely (or potential) lower energy counterparts.
Approximately 750 UGSs have been observed with >4ks of observations
each.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table2.dat 105 17 Swift X-Ray sources with extreme photon indices,
alphaX<0 or alphaX>4,
included in the main analysis
table3.dat 117 218 Parameters for singlet X-ray sources detected
with Swift-XRT within 4FGL-DR4 unassociated
source uncertainty ellipses
table4.dat 117 70 Parameters for X-Ray sources detected with
Swift-XRT within 4FGL-DR4 unassociated
source uncertainty ellipses
table5.dat 114 218 Neural network classification of X-Ray singlets
utilizing the blazar probability parameter
table6.dat 113 70 Neural network classification of X-Ray multiplets
utilizing the blazar probability parameter
table7.dat 88 218 Potential radio counterparts to X-ray singlets
table8.dat 88 70 Potential radio counterparts to X-ray multiplets
table9a.dat 377 218 Targets with a single significant X-ray sources
table9b.dat 372 70 Targets with multiple significant X-ray sources
--------------------------------------------------------------------------------
See also:
B/swift : Swift Master Catalog (HEASARC, 2004-)
J/ApJS/208/17 : 2nd Fermi LAT Cat. of gamma-ray pulsars (2PC)
(Abdo+, 2013)
J/ApJS/247/33 : The Fermi LAT fourth source catalog (4FGL)
(Abdollahi+, 2020)
J/ApJS/255/30 : VLASS QL Ep.1 Catalog, CIRADA version (Gordon+, 2021)
J/ApJ/923/75 : X-ray/UV/opt counterparts of 4FGL gamma-ray sources
(Kerby+, 2021)
J/other/PASA/38.58 : Rapid ASKAP Continuum Survey. II. RACS Cat. (Hale+, 2021)
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 13 A13 --- 4FGL 4FGL designation, JHHMM.m+DDMMa
14 A1 --- n_4FGL [*] * indicates 4FGL sources with multiple
X-ray sources
16- 31 A16 --- SwXF4-DR4 SwXF4-DR4 designation, JHHMMSS.s+DDMMSS
33- 36 F4.2 10+22cm-2 nH Hydrogen column density obtained with the
nH function
38- 42 F5.2 --- alphaX Photon index (1)
44- 47 F4.2 --- e_alphaX ? Photon index error (1)
49-107 A59 --- Ctp Possible Counterpart (2)
--------------------------------------------------------------------------------
Note (1): Photon indices of 10 are values given when XSPEC failed to fit the
spectral parameters.
Note (2): Possible source counterparts include classification and Gaia G-band
magnitude acquired from SIMBAD if available.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table3.dat table4.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 13 A13 --- 4FGL Target AFGL identifier, JHHMM.m+DDMM (1)
15- 20 F6.2 [mW/m2] logFG ?=- log γ-ray 0.1-100GeV flux
(erg/s/cm2)
22- 25 F4.2 --- PLIndex ?=- gamma-ray photon index when fitting with
PowerLaw
27- 30 F4.2 [-] logVIndex ?=- Year-to-year γ-ray variability
32- 36 F5.2 [-] logPLECSigC ?=- Curvature significance of PLEC model
38- 53 A16 --- SwXF4-DR4 XRT designation, JHHMMSS.s+DDMMSS, SwXF4-DR4
55- 70 A16 --- SwUVF4-DR4 UVOT designation, JHHMMSS.s+DDMMSS,
SwUVF4-DR4
72- 76 F5.2 --- S/N XRT signal-to-noise ratio
78- 83 F6.2 [mW/m2] logFX Unabsorbed log X-ray 0.3-10keV flux
(erg/s/cm2) (2)
85- 89 F5.2 --- alphaX X-ray photon index (2)
91- 93 A3 ---- Filter UVOT filter used to calculate Vmag
95 A1 --- l_Vmag Limit flag on V-band
96-100 F5.2 mag Vmag ?=- V-band magnitude calculated with
UVOT filter (3)
102-117 A16 --- SwXF4-DR4K SwXF4-DR4 identifier, JHHMMSS.s+DDMMSS,
Kerby et al. (2021ApJ...923...75K 2021ApJ...923...75K,
Cat. J/ApJ/923/75K) (4)
--------------------------------------------------------------------------------
Note (1): Data are acquired using publicly available Fermi-LAT, Swift-XRT, and
Swift-UVOT data. Gamma-ray fluxes are from 100 MeV to 100 GeV
(extttEnergy_Flux100; S. Abdollahi et al. 2020).
Note (2): Derivation of X-ray (unabsorbed X-ray flux, F_X, and X-ray photon
index, alpha_X) and UV/optical (V-band magnitude, m_V) parameters is described
in Section 3.
Note (3): If a source has UVOT observations but no detections, a limiting
magnitude is calculated for m_V using the labeled filter's observation.
If no data is given for filter or m_V, the source lacks UVOT observations.
Note (4): SwXF4 describes the alternative alias of the X-ray sources described
in Kerby et al. (2021ApJ...923...75K 2021ApJ...923...75K, Cat. J/ApJ/923/75), where an ellipsis
represents a new X-ray source.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table5.dat table6.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 A2 --- Type Type, only in table5.dat (1)
4- 16 A13 --- 4FGL 4FGL name (JHHMM.m+DDMMa)
18 A1 --- SGU [*] * for gamma-ray sources flagged as soft
Galactic unassociated (SGU) in 4FGL-DR4
20- 35 A16 --- SwXF4-DR4 SwXF4-DR4 name (JHHMMSS.s+DDMMSS)
37- 42 F6.4 --- Pbzr ?=- Blazar probability parameter
44- 49 F6.4 --- Plim ?=- For X-ray sources with a V-magnitude
lower limit, classification results
if mV=30
51- 56 F6.4 --- PK21 ?=- Blazar probability for X-ray sources
shared with Kerby et al.
(2021ApJ...923...75K 2021ApJ...923...75K, Cat. J/ApJ/923/75)
58- 85 A28 --- SIM-src Potential SIMBAD counterpart
86 A1 --- n_SIM-src [*] Note on SIM-src (2)
88- 91 F4.2 arcsec SIM-dist ? Offset for potential SIMBAD counterpart
93-114 A22 --- Class Literature classification
--------------------------------------------------------------------------------
Note (1): This paper classification as follws:
LP = Likely Pulsars (Pbzr<0.01)
LB = Likely Blazars (Pbzr>0.99)
A = Ambiguous (0.01<Pbzr<0.99 or Pbzr=NaN)
Note (2): * for X-ray sources with multiple potential SIMBAD
counterparts within 5".
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table7.dat table8.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 13 A13 --- 4FGL AFGL identifier, JHHMM.m+DDMM
15- 30 A16 --- SwXF4-DR4 SwXF4-DR4 name (JHHMMSS.s+DDMMSS)
32 A1 --- l_Vmag Limit flag on Vmag
33- 37 D5.2 mag Vmag ?=- V-band magnitude
39- 70 A32 --- RName Radio Source name
71- 74 F4.2 arcsec Offset ? Offset betweeb gamma and radio sources
76- 82 F7.2 mJy Frad ? Radio flux density
84 A1 --- l_R Limit flag on R
85- 88 I4 --- R ? Radio-loudness parameter (1)
--------------------------------------------------------------------------------
Note (1): Radio-loudness parameter, R, calculated with optical flux density
in units of erg/s/cm2/Hz.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table9a.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 F7.3 deg RAdeg Right ascension in decimal degrees (J2000)
9- 15 F7.3 deg DEdeg Declination in decimal degrees (J2000)
17- 29 A13 --- 4FGL Target 4FGL identifier
31- 36 F6.2 [mW/m2] logFG Log γ-ray 0.1-100GeV flux
(erg/s/cm2) (1)
38- 41 F4.2 [mW/m2] e_logFG Uncertainty in logFG (erg/s/cm2) (1)
43- 47 F5.3 --- PLIndex Gamma-ray photon index when fitting with
PowerLaw (1)
49- 55 F7.3 --- VIndex Year-to-year γ-ray variability (1)
57- 61 F5.3 --- PLECSigC Curvature significance of PLEC model (1)
63 I1 --- SGU Soft Galactic Unassociated source flag (1)
65- 80 A16 --- SwXF4-DR4 Target SwXF4-DR4 identifier
82- 97 A16 --- SwUVF4-DR4 Target SwUVF4-DR4 identifier
99-102 F4.2 arcsec erad Positional XRT uncertainty
104-109 F6.1 ct Counts ? Background subtracted X-ray counts of
XRT source
111-115 F5.2 --- SNSOSTA SOSTA XRT Signal-to-Noise
117-122 F6.2 [mW/m2] logFX Unabsorbed log X-ray 0.3-10keV flux
(erg/s/cm2)
124-127 F4.2 [mW/m2] e_logFX Uncertainty in logFX (erg/s/cm2)
129-133 F5.2 --- alphaX X-ray photon index
135-139 F5.2 --- e_alphaX Uncertainty in PhoInd
141-142 A2 --- UVOT Swift-UVOT filter used for Vmag
144 A1 --- f_UVOT Flag on UVOT (2)
146-150 F5.2 mag Vmag ? Apparent V band AB magnitude estimate (3)
152 A1 --- l_Vmag [l] l for detection limiting V magnitude
153-156 F4.2 mag e_Vmag ? Uncertainty in Vmag (3)
158-162 F5.2 mag vvmag ? Apparent vv band (547nm) AB magnitude
measurement (3)
164 A1 --- l_vvmag [l] l for detection limiting vv magnitude
165-168 F4.2 mag e_vvmag ? Uncertainty in vvmag (3)
170-174 F5.2 mag bbmag ? Apparent bb band (439nm) AB magnitude
measurement (3)
176 A1 --- l_bbmag [l] l for detection limiting bb magnitude
177-180 F4.2 mag e_bbmag ? Uncertainty in bbmag (3)
182-186 F5.2 mag uumag ? Apparent uu band (347nm) AB magnitude
measurement (3)
188 A1 --- l_uumag [l] l for detection limiting uu magnitude
189-192 F4.2 mag e_uumag ? Uncertainty in uumag (3)
194-198 F5.2 mag w1mag ? Apparent w1 band (260nm) AB magnitude
measurement (3)
200 A1 --- l_w1mag [l] l for detection limiting w1 magnitude
201-204 F4.2 mag e_w1mag ? Uncertainty in w1mag (3)
206-210 F5.2 mag m2mag ? Apparent m2 (225nm) band AB magnitude
measurement (3)
212 A1 --- l_m2mag [l] l for detection limiting m2 magnitude
213-216 F4.2 mag e_m2mag ? Uncertainty in m2mag (3)
218-222 F5.2 mag w2mag ? Apparent w2 band (193nm) AB magnitude
measurement (3)
224 A1 --- l_w2mag [l] l for detection limiting w2 magnitude
225-228 F4.2 mag e_w2mag ? Uncertainty in w2mag (3)
230-235 F6.4 --- Pbzr ? NNC blazar probability
237-242 F6.4 --- Plim ? Blazar probability limit, mV=30 (4)
244-259 A16 --- SwXF4-DR4K Target SwXF4-DR4 identifier (5)
261-266 F6.4 --- PK21 ? Blazar probability (5)
268-295 A28 --- SIM-src Likely counterpart in SIMBAD
297 A1 --- f_SIM-src Flag on SIM-src (6)
299-302 F4.2 arcsec SIM-dist ? Offset from SIMBAD counterpart
304-325 A22 --- SIM-type SIMBAD counterpart classification
327-357 A31 --- Rad-src Likely radio counterpart (7)
359-362 F4.2 arcsec Rad-dist ? Offset from radio counterpart
364-370 F7.2 mJy FRad ? Radio counterpart radio flux density (8)
372-375 I4 --- Rloud Radio loudness parameter
377 I1 --- l_Rloud [0/1] Radio loudness parameter lower limit
flag (9)
--------------------------------------------------------------------------------
Note (1): Parameters are from the data columns in 4FGL-DR4 (Ballet et al..
2023arXiv230712546B 2023arXiv230712546B).
Note (2): Flag as follows:
* = Filter used an optical spectral slope of 1.5.
Note (3): Magnitude uncertainties of -1.00 are detection limiting magnitudes.
Blank values imply no viable overlapping observation of target.
Note (4): Blank values imply UVOT source was detected.
Note (5): From Kerby et al. (2021ApJ...923...75K 2021ApJ...923...75K, Cat. J/ApJ/923/75).
Note (6): Flag as follows:
* = Source has multiple nearby potential sources.
Note (7): Based on VLASS and RACS catalogs with preference for VLASS if source
detected in both.
Note (8): VLASS band is 2-4GHz and RACS band is 0.75-1.5GHz.
Note (9): lower limit flag ad follows:
1 = Rloud is a lower limit.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table9b.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 F7.3 deg RAdeg Right ascension in decimal degrees (J2000)
9- 15 F7.3 deg DEdeg Declination in decimal degrees (J2000)
17- 29 A13 --- 4FGL Target 4FGL identifier
31- 36 F6.2 [mW/m2] logFG Log γ-ray 0.1-100GeV flux
(erg/s/cm2) (1)
38- 41 F4.2 [mW/m2] e_logFG Uncertainty in logFG (erg/s/cm2) (1)
43- 47 F5.3 --- PLIndex Gamma-ray photon index when fitting with
PowerLaw (1)
49- 55 F7.3 --- VIndex Year-to-year γ-ray variability (1)
57- 61 F5.3 --- PLECSigC Curvature significance of PLEC model (1)
63 I1 --- SGU Soft Galactic Unassociated source flag (1)
65- 80 A16 --- SwXF4-DR4 Target SwXF4-DR4 identifier
82- 97 A16 --- SwUVF4-DR4 Target SwUVF4-DR4 identifier
99-102 F4.2 arcsec erad Positional XRT uncertainty
104-108 F5.1 ct counts Background subtracted X-ray counts of
XRT source
110-114 F5.2 --- SNSOSTA SOSTA XRT Signal-to-Noise
116-121 F6.2 [mW/m2] logFX Unabsorbed log X-ray 0.3-10keV flux
(erg/s/cm2)
123-126 F4.2 [mW/m2] e_logFX Uncertainty in logFX (erg/s/cm2)
128-132 F5.2 --- alphaX X-ray photon index
134-137 F4.2 --- e_alphaX Uncertainty in PhoInd
139-140 A2 --- UVOT Swift-UVOT filter used for Vmag
142 A1 --- f_UVOT Flag on UVOT (2)
144-148 F5.2 mag Vmag ? Apparent V band AB magnitude estimate (3)
150 A1 --- l_Vmag [l] l for detection limiting V magnitude
151-154 F4.2 mag e_Vmag ? Uncertainty in Vmag (3)
156-160 F5.2 mag vvmag ? Apparent vv band AB magnitude
measurement (3)
162 A1 --- l_vvmag [l] l for detection limiting vv magnitude
163-166 F4.2 mag e_vvmag ? Uncertainty in vvmag (3)
168-172 F5.2 mag bbmag ? Apparent bb band AB magnitude
measurement (3)
174 A1 --- l_bbmag [l] l for detection limiting bb magnitude
175-178 F4.2 mag e_bbmag ? Uncertainty in bbmag (3)
180-184 F5.2 mag uumag ? Apparent uu band AB magnitude
measurement (3)
186 A1 --- l_uumag [l] l for detection limiting uu magnitude
187-190 F4.2 mag e_uumag ? Uncertainty in uumag (3)
192-196 F5.2 mag w1mag ? Apparent w1 band AB magnitude
measurement (3)
198 A1 --- l_w1mag [l] l for detection limiting w1 magnitude
199-202 F4.2 mag e_w1mag ? Uncertainty in w1mag (3)
204-208 F5.2 mag m2mag ? Apparent m2 band AB magnitude
measurement (3)
210 A1 --- l_m2mag [l] l for detection limiting m2 magnitude
211-214 F4.2 mag e_m2mag ? Uncertainty in m2mag (3)
216-220 F5.2 mag w2mag ? Apparent w2 band AB magnitude
measurement (3)
222 A1 --- l_w2mag [l] l for detection limiting w2 magnitude
223-226 F4.2 mag e_w2mag ? Uncertainty in w2mag (3)
228-233 F6.4 --- Pbzr ? NNC blazar probability
235-240 F6.4 --- Plim ? Blazar probability limit, mV=30 (4)
242-257 A16 --- SwXF4-DR4K Target SwXF4 identifier (5)
259-264 F6.4 --- PK21 ? Blazar probability (5)
266-293 A28 --- SIM-src Likely counterpart in SIMBAD
295 A1 --- f_SIM-src Flag on SIM-src (6)
297-300 F4.2 arcsec SIM-dist ? Offset from SIMBAD counterpart
302-322 A21 --- SIM-type SIMBAD counterpart classification
324-354 A31 --- Rad-src Likely radio counterpart (7)
356-359 F4.2 arcsec Rad-dist ? Offset from radio counterpart
361-365 F5.2 mJy FRad ? Radio counterpart radio flux density (8)
367-370 I4 --- Rloud Radio loudness parameter
372 I1 --- l_Rloud [0/1] Radio loudness parameter lower limit
flag (9)
--------------------------------------------------------------------------------
Note (1): Parameters are from the data columns in 4FGL-DR4, Ballet et al.,
2023arXiv230712546B 2023arXiv230712546B.
Note (2): Flag as follows:
* = Filter used an optical spectral slope of 1.5.
Note (3): Magnitude uncertainties of -1.00 are detection limiting magnitudes.
Blank values imply no viable overlapping observation of target.
Note (4): Blank values imply UVOT source was detected.
Note (5): From Kerby et al. (2021ApJ...923...75K 2021ApJ...923...75K, Cat. J/ApJ/923/75).
Note (6): Flag as follows:
* = Source has multiple nearby potential sources.
Note (7): Based on VLASS and RACS catalogs with preference for VLASS if source
detected in both.
Note (8): VLASS band is 2-4GHz and RACS band is 0.75-1.5GHz.
Note (9): lower limit flag ad follows:
1 = Rloud is a lower limit
--------------------------------------------------------------------------------
Acknowledgements:
From Kyle Neumann, kdn5172(at)psu.edu
The authors would like to express their gratitude to the Swift Science
Operations Team for their dedicated efforts in scheduling the Swift
observations of the 4FGL unassociated gamma-ray sources.
This research has made use of data and/or softwareprovided by the High
Energy Astrophysics Science Archive Research Center (HEASARC), which
is a service of the Astrophysics Science Division at NASA/GSFC. We
gratefully acknowledge the support of NASA grants 80NSSC20K1526 and
80NSSC22K1588, and J.E.G. is supported for Fermi-LAT work under NASA
contract S-15633Y.
License: CC-BY-4.0
(End) Patricia Vannier [CDS] 10-Jun-2026