J/ApJ/999/244ivo://CDS.VizieR/J/ApJ/999/244CDSNeumann K.D.Falcone A.D.Dikerby S.Deppe S.Ferrara E.C.,Kennea J.A.Cenko B.Grove E.2026-06-12T13:45:41Z2026-06-11T13:44:12ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frgamma-ray-astronomyx-ray-sourcesvisible-astronomybroad-band-photometryactive-galactic-nucleibl-lacertae-objectspulsarsOur 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, 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 (P_bzr_>0.99) and six as likely pulsars (P_bzr_<0.01), with 34 sources yielding ambiguous results. Including 70 multiplet X-ray sources, we increase the number of P_bzr_>0.99 sources to 227 and P_bzr_<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.2026ApJ...999..244Nhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/ApJ/999/244CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toB/swift : Swift Master Catalog (HEASARC, 2004-)J/ApJS/208/17 : 2nd Fermi LAT Cat. of gamma-ray pulsars (2PC)J/ApJS/247/33 : The Fermi LAT fourth source catalog (4FGL)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 sourcesJ/other/PASA/38.58 : Rapid ASKAP Continuum Survey. II. RACS Cat. (Hale+, 2021)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/ApJ/999/244https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/ApJ/999/244http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/ApJ/999/244https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/ApJ/999/244https://vizier.iucaa.in/viz-bin/votable?-source=J/ApJ/999/244http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/ApJ/999/244GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/ApJ/999/244/table9a (Targets with a single significant X-ray sources)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/ApJ/999/244/table9a?https://vizier.iucaa.in/viz-bin/conesearch/J/ApJ/999/244/table9a?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/ApJ/999/244/table9a?GETtext/xml+votable180.050000true1.142-40.010.005555555555555556Cone search capability for table J/ApJ/999/244/table9b (Targets with multiple significant X-ray sources)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/ApJ/999/244/table9b?https://vizier.iucaa.in/viz-bin/conesearch/J/ApJ/999/244/table9b?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/ApJ/999/244/table9b?GETtext/xml+votable180.050000true4.286-46.1020.0055555555555555566/50 760 836 973 1113 1153 1354 1531 1835 2142 2167 2324 2886 3105 3333 3619 3721 3793 4376 5193 5415 5421 5583 5800 6764 7027 7092 7137 7237 7531 7743 7940 7958 8603 8733 9321 9379 9475 9642 9759 9877 9914 10011 10468 11156 11265 11964 12164 12668 12730 12856 12923 13097 13147 13273 13337 13370 13452 13765 13908 13986 14263 14274 14311 14462 14619 14856 15043 15183 15640 16004 16044 16137 16182 16226 16231 16284 16385 16541 16560 16648-16650 16880-16881 17223 17270 17734 18333 18343 18409 18626 18815 19084 19431 20221 20381 20588 21385 21496 21710 21800 21903 22341 23427 23728 23837 24433 24554 24726 24789 25262 25403 25512 25525 25693 25974 26933 27074 27155 27964 28172 28837 28848-28849 28855 28876 29042 29151 29374 29533 29547 29881 30024 30168 30196 30423 30425 30624 31720 31868 32379 32394 32420 32742 32909 33206 33341 33482 33514 33596 33801 34053 34216 34386 34860 34871 34949 35343 35374 35395 35482 35637 35873 36128 36394 36445 36484 36901 37222 37233 37365 37629 37744 37872-37873 37884 37931 38055 38092 38195 38492 38837 39114 39139 39259 39276 39431 39439 39459 39792 39828 39882 40025 40164 40289 40366 41253 41631 41655 41688 41696 42087 42101 42172 42216 42226 42342 42378-42379 42392 42397 42630 42635 42804 42989 43197 43278 43350 43600 43751 44634 44741 44751 44986 45077 45079 45375 45492 45707 45754 45859 45904 45974 45980 45998 46209 46444 46480 46622 47245 47392 48200 48707 48898 490803.377399441402775e-19 1.0292464363059578e-18https://cdsarc.cds.unistra.fr/viz-bin/moc/J/ApJ/999/244?format=asciiGamma-rayX-rayOpticalRadiodefaultJ/ApJ/999/244/table2Swift X-Ray sources with extreme photon indices, alpha_X_<0 or alpha_X_>4, included in the main analysisrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordint4FGL4FGL designation, JHHMM.m+DDMMameta.id;meta.maincharn_4FGL[*] * indicates 4FGL sources with multiple X-ray sourcesmeta.notecharSwXF4-DR4SwXF4-DR4 designation, JHHMMSS.s+DDMMSSmeta.idcharnHHydrogen column density obtained with the nH function1e+22cm**-2phys.columnDensityfloatalphaXPhoton index (1)stat.fit.paramfloate_alphaX? Photon index error (1)stat.errorfloatnullableCtpPossible Counterpart (2)meta.id.assoccharJ/ApJ/999/244/table3Parameters for singlet X-ray sources detected with Swift-XRT within 4FGL-DR4 unassociated source uncertainty ellipsesrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordint4FGLTarget AFGL identifier, JHHMM.m+DDMM (1)meta.id;meta.maincharlogFG? log {gamma}-ray 0.1-100GeV flux (erg/s/cm2)log(mW.m**-2)phot.flux.density;em.gamma.hardfloatnullablePLIndex? gamma-ray photon index when fitting with PowerLawstat.fit.paramfloatnullablelogVIndex? Year-to-year {gamma}-ray variabilitysrc.var.indexfloatnullablelogPLECSigC? Curvature significance of PLEC modelstat.fit.paramfloatnullableSwXF4-DR4XRT designation, JHHMMSS.s+DDMMSS, SwXF4-DR4meta.idcharSwUVF4-DR4UVOT designation, JHHMMSS.s+DDMMSS, SwUVF4-DR4meta.idcharS/NXRT signal-to-noise ratiostat.snrfloatlogFXUnabsorbed log X-ray 0.3-10keV flux (erg/s/cm2) (2)log(mW.m**-2)phot.flux;em.X-rayfloatalphaXX-ray photon index (2)stat.fit.paramfloatFilterUVOT filter used to calculate Vmaginstr.setup;instr.filtercharl_VmagLimit flag on V-bandmeta.code.errorcharVmag? V-band magnitude calculated with UVOT filter (3)magphot.mag;em.opt.VfloatnullableSwXF4-DR4KSwXF4-DR4 identifier, JHHMMSS.s+DDMMSS, Kerby et al. (2021ApJ...923...75K, Cat. J/ApJ/923/75K) (4)meta.idcharJ/ApJ/999/244/table4Parameters for X-Ray sources detected with Swift-XRT within 4FGL-DR4 unassociated source uncertainty ellipsesPLIndex? gamma-ray photon index when fitting with PowerLawstat.fit.paramfloatnullablelogVIndex? Year-to-year {gamma}-ray variabilitysrc.var.indexfloatnullablelogPLECSigC? Curvature significance of PLEC modelstat.fit.paramfloatnullableSwXF4-DR4XRT designation, JHHMMSS.s+DDMMSS, SwXF4-DR4meta.idcharSwUVF4-DR4UVOT designation, JHHMMSS.s+DDMMSS, SwUVF4-DR4meta.idcharS/NXRT signal-to-noise ratiostat.snrfloatlogFXUnabsorbed log X-ray 0.3-10keV flux (erg/s/cm2) (2)log(mW.m**-2)phot.flux;em.X-rayfloatalphaXX-ray photon index (2)stat.fit.paramfloatFilterUVOT filter used to calculate Vmaginstr.setup;instr.filtercharl_VmagLimit flag on V-bandmeta.code.errorcharVmag? V-band magnitude calculated with UVOT filter (3)magphot.mag;em.opt.VfloatnullableSwXF4-DR4KSwXF4-DR4 identifier, JHHMMSS.s+DDMMSS, Kerby et al. (2021ApJ...923...75K, Cat. J/ApJ/923/75K) (4)meta.idcharrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordint4FGLTarget AFGL identifier, JHHMM.m+DDMM (1)meta.id;meta.maincharlogFG? log {gamma}-ray 0.1-100GeV flux (erg/s/cm2)log(mW.m**-2)phot.flux.density;em.gamma.hardfloatnullableJ/ApJ/999/244/table5Neural network classification of X-Ray singlets utilizing the blazar probability parameterrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintTypeType, only in table5.dat (1)meta.note;src.morph.typechar4FGL4FGL name (JHHMM.m+DDMMa)meta.id;meta.maincharSGU[*] * for gamma-ray sources flagged as soft Galactic unassociated (SGU) in 4FGL-DR4meta.codecharSwXF4-DR4SwXF4-DR4 name (JHHMMSS.s+DDMMSS)meta.idcharPbzr? Blazar probability parameterstat.fit.goodnessfloatnullablePlim? For X-ray sources with a V-magnitude lower limit, classification results if m_V_=30stat.fit.goodnessfloatnullablePK21? Blazar probability for X-ray sources shared with Kerby et al. (2021ApJ...923...75K, Cat. J/ApJ/923/75)stat.fit.goodnessfloatnullableSIM-srcPotential SIMBAD counterpartmeta.id.assoccharn_SIM-src[*] Note on SIM-src (2)meta.notecharSIM-dist? Offset for potential SIMBAD counterpartarcsecpos.distancefloatnullableClassLiterature classificationsrc.classcharJ/ApJ/999/244/table6Neural network classification of X-Ray multiplets utilizing the blazar probability parameterSwXF4-DR4SwXF4-DR4 name (JHHMMSS.s+DDMMSS)meta.idcharPbzr? Blazar probability parameterstat.fit.goodnessfloatnullablerecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintTypeType, only in table5.dat (1)meta.note;src.morph.typechar4FGL4FGL name (JHHMM.m+DDMMa)meta.id;meta.maincharSGU[*] * for gamma-ray sources flagged as soft Galactic unassociated (SGU) in 4FGL-DR4meta.codecharPlim? For X-ray sources with a V-magnitude lower limit, classification results if m_V_=30stat.fit.goodnessfloatnullablePK21? Blazar probability for X-ray sources shared with Kerby et al. (2021ApJ...923...75K, Cat. J/ApJ/923/75)stat.fit.goodnessfloatnullableSIM-srcPotential SIMBAD counterpartmeta.id.assoccharn_SIM-src[*] Note on SIM-src (2)meta.notecharSIM-dist? Offset for potential SIMBAD counterpartarcsecpos.distancefloatnullableClassLiterature classificationsrc.classcharJ/ApJ/999/244/table7Potential radio counterparts to X-ray singletsrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordint4FGLAFGL identifier, JHHMM.m+DDMMmeta.id;meta.maincharSwXF4-DR4SwXF4-DR4 name (JHHMMSS.s+DDMMSS)meta.idcharl_VmagLimit flag on Vmagmeta.code.errorcharVmag? V-band magnitudemagphot.mag;em.opt.VdoublenullableRNameRadio Source namemeta.id.assoccharOffset? Offset betweeb gamma and radio sourcesarcsecpos.distancefloatnullableFrad? Radio flux densitymJyphot.flux.density;em.radiofloatnullablel_RLimit flag on Rmeta.code.errorcharR? Radio-loudness parameter (1)phot.flux;arith.ratiointnullableJ/ApJ/999/244/table8Potential radio counterparts to X-ray multipletsrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordint4FGLAFGL identifier, JHHMM.m+DDMMmeta.id;meta.maincharSwXF4-DR4SwXF4-DR4 name (JHHMMSS.s+DDMMSS)meta.idcharl_VmagLimit flag on Vmagmeta.code.errorcharVmag? V-band magnitudemagphot.mag;em.opt.VdoublenullableRNameRadio Source namemeta.id.assoccharOffset? Offset betweeb gamma and radio sourcesarcsecpos.distancefloatnullableFrad? Radio flux densitymJyphot.flux.density;em.radiofloatnullablel_RLimit flag on Rmeta.code.errorcharR? Radio-loudness parameter (1)phot.flux;arith.ratiointnullableJ/ApJ/999/244/table9aTargets with a single significant X-ray sourcese_alphaXUncertainty in PhoIndstat.errorfloatUVOTSwift-UVOT filter used for Vmagmeta.code;instr.filtercharf_UVOTFlag on UVOT (2)meta.codecharVmag? Apparent V band AB magnitude estimate (3)magphot.mag;em.opt.Vfloatnullablel_Vmag[l] l for detection limiting V magnitudemeta.code.errorcharrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintRAJ2000Right ascension in decimal degrees (J2000)degpos.eq.ra;meta.mainDEJ2000Declination in decimal degrees (J2000)degpos.eq.dec;meta.main4FGLTarget 4FGL identifiermeta.id;meta.maincharlogFGLog {gamma}-ray 0.1-100GeV flux (erg/s/cm2) (1)log(mW.m**-2)phot.flux.density;em.gamma.hardfloate_logFGUncertainty in logFG (erg/s/cm2) (1)log(mW.m**-2)stat.errorfloatPLIndexGamma-ray photon index when fitting with PowerLaw (1)stat.fit.paramfloatVIndexYear-to-year {gamma}-ray variability (1)src.var.indexfloatPLECSigCCurvature significance of PLEC model (1)stat.fit.paramfloatSGUSoft Galactic Unassociated source flag (1)meta.codeintSwUVF4-DR4Target SwUVF4-DR4 identifiermeta.idchareradPositional XRT uncertaintyarcsecstat.error;posfloatCounts? Background subtracted X-ray counts of XRT sourcectphot.count;em.X-rayfloatnullableSNSOSTASOSTA XRT Signal-to-Noisestat.snrfloatlogFXUnabsorbed log X-ray 0.3-10keV flux (erg/s/cm2)log(mW.m**-2)phot.flux;em.X-rayfloate_logFXUncertainty in logFX (erg/s/cm2)log(mW.m**-2)stat.errorfloatalphaXX-ray photon indexstat.fit.paramfloate_Vmag? Uncertainty in Vmag (3)magstat.error;phot.magfloatnullablevvmag? Apparent vv band (547nm) AB magnitude measurement (3)magphot.mag;em.opt.Vfloatnullablel_vvmag[l] l for detection limiting vv magnitudemeta.code.errorchare_vvmag? Uncertainty in vvmag (3)magstat.error;phot.magfloatnullablebbmag? Apparent bb band (439nm) AB magnitude measurement (3)magphot.mag;em.opt.Bfloatnullablel_bbmag[l] l for detection limiting bb magnitudemeta.code.errorchare_bbmag? Uncertainty in bbmag (3)magstat.error;phot.magfloatnullableuumag? Apparent uu band (347nm) AB magnitude measurement (3)magphot.mag;em.opt.Ufloatnullablel_uumag[l] l for detection limiting uu magnitudemeta.code.errorchare_uumag? Uncertainty in uumag (3)magstat.error;phot.magfloatnullablew1mag? Apparent w1 band (260nm) AB magnitude measurement (3)magphot.mag;em.UVfloatnullablel_w1mag[l] l for detection limiting w1 magnitudemeta.code.errorchare_w1mag? Uncertainty in w1mag (3)magstat.error;phot.magfloatnullablem2mag? Apparent m2 (225nm) band AB magnitude measurement (3)magphot.mag;em.UVfloatnullablel_m2mag[l] l for detection limiting m2 magnitudemeta.code.errorchare_m2mag? Uncertainty in m2mag (3)magstat.error;phot.magfloatnullablew2mag? Apparent w2 band (193nm) AB magnitude measurement (3)magphot.mag;em.UVfloatnullablel_w2mag[l] l for detection limiting w2 magnitudemeta.code.errorchare_w2mag? Uncertainty in w2mag (3)magstat.error;phot.magfloatnullablePbzr? NNC blazar probabilitystat.fit.goodnessfloatnullablePlim? Blazar probability limit, mV=30 (4)stat.fit.goodnessfloatnullableSwXF4-DR4KTarget SwXF4-DR4 identifier (5)meta.idcharPK21? Blazar probability (5)stat.fit.goodnessfloatnullableSIM-srcLikely counterpart in SIMBADmeta.id.assoccharf_SIM-srcFlag on SIM-src (6)meta.codecharSIM-dist? Offset from SIMBAD counterpartarcsecpos.distancefloatnullableSIM-typeSIMBAD counterpart classificationsrc.classcharRad-srcLikely radio counterpart (7)meta.id.assoccharRad-dist? Offset from radio counterpartarcsecpos.distancefloatnullableFRad? Radio counterpart radio flux density (8)mJyphot.flux.density;em.radiofloatnullableRloudRadio loudness parameterphot.flux;arith.ratiointl_Rloud[0/1] Radio loudness parameter lower limit flag (9)meta.code.errorintSwXF4-DR4Target SwXF4-DR4 identifiermeta.idcharJ/ApJ/999/244/table9bTargets with multiple significant X-ray sourcesPLIndexGamma-ray photon index when fitting with PowerLaw (1)stat.fit.paramfloatVIndexYear-to-year {gamma}-ray variability (1)src.var.indexfloatl_Rloud[0/1] Radio loudness parameter lower limit flag (9)meta.code.errorintrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintRAJ2000Right ascension in decimal degrees (J2000)degpos.eq.ra;meta.mainDEJ2000Declination in decimal degrees (J2000)degpos.eq.dec;meta.main4FGLTarget 4FGL identifiermeta.id;meta.maincharlogFGLog {gamma}-ray 0.1-100GeV flux (erg/s/cm2) (1)log(mW.m**-2)phot.flux.density;em.gamma.hardfloate_logFGUncertainty in logFG (erg/s/cm2) (1)log(mW.m**-2)stat.errorfloatPLECSigCCurvature significance of PLEC model (1)stat.fit.paramfloatSGUSoft Galactic Unassociated source flag (1)meta.codeintSwUVF4-DR4Target SwUVF4-DR4 identifiermeta.idchareradPositional XRT uncertaintyarcsecstat.error;posfloatcountsBackground subtracted X-ray counts of XRT sourcectphot.count;em.X-rayfloatSNSOSTASOSTA XRT Signal-to-Noisestat.snrfloatlogFXUnabsorbed log X-ray 0.3-10keV flux (erg/s/cm2)log(mW.m**-2)phot.flux;em.X-rayfloate_logFXUncertainty in logFX (erg/s/cm2)log(mW.m**-2)stat.errorfloatalphaXX-ray photon indexstat.fit.paramfloate_alphaXUncertainty in PhoIndstat.errorfloatUVOTSwift-UVOT filter used for Vmagmeta.code;instr.filtercharf_UVOTFlag on UVOT (2)meta.codecharVmag? Apparent V band AB magnitude estimate (3)magphot.mag;em.opt.Vfloatnullablel_Vmag[l] l for detection limiting V magnitudemeta.code.errorchare_Vmag? Uncertainty in Vmag (3)magstat.error;phot.magfloatnullablevvmag? Apparent vv band AB magnitude measurement (3)magphot.mag;em.opt.Vfloatnullablel_vvmag[l] l for detection limiting vv magnitudemeta.code.errorchare_vvmag? Uncertainty in vvmag (3)magstat.error;phot.magfloatnullablebbmag? Apparent bb band AB magnitude measurement (3)magphot.mag;em.opt.Bfloatnullablel_bbmag[l] l for detection limiting bb magnitudemeta.code.errorchare_bbmag? Uncertainty in bbmag (3)magstat.error;phot.magfloatnullableuumag? Apparent uu band AB magnitude measurement (3)magphot.mag;em.opt.Ufloatnullablel_uumag[l] l for detection limiting uu magnitudemeta.code.errorchare_uumag? Uncertainty in uumag (3)magstat.error;phot.magfloatnullablew1mag? Apparent w1 band AB magnitude measurement (3)magphot.mag;em.UVfloatnullablel_w1mag[l] l for detection limiting w1 magnitudemeta.code.errorchare_w1mag? Uncertainty in w1mag (3)magstat.error;phot.magfloatnullablem2mag? Apparent m2 band AB magnitude measurement (3)magphot.mag;em.UVfloatnullablel_m2mag[l] l for detection limiting m2 magnitudemeta.code.errorchare_m2mag? Uncertainty in m2mag (3)magstat.error;phot.magfloatnullablew2mag? Apparent w2 band AB magnitude measurement (3)magphot.mag;em.UVfloatnullablel_w2mag[l] l for detection limiting w2 magnitudemeta.code.errorchare_w2mag? Uncertainty in w2mag (3)magstat.error;phot.magfloatnullablePbzr? NNC blazar probabilitystat.fit.goodnessfloatnullablePlim? Blazar probability limit, mV=30 (4)stat.fit.goodnessfloatnullableSwXF4-DR4KTarget SwXF4 identifier (5)meta.idcharPK21? Blazar probability (5)stat.fit.goodnessfloatnullableSIM-srcLikely counterpart in SIMBADmeta.id.assoccharf_SIM-srcFlag on SIM-src (6)meta.codecharSIM-dist? Offset from SIMBAD counterpartarcsecpos.distancefloatnullableSIM-typeSIMBAD counterpart classificationsrc.classcharRad-srcLikely radio counterpart (7)meta.id.assoccharRad-dist? Offset from radio counterpartarcsecpos.distancefloatnullableFRad? Radio counterpart radio flux density (8)mJyphot.flux.density;em.radiofloatnullableRloudRadio loudness parameterphot.flux;arith.ratiointSwXF4-DR4Target SwXF4-DR4 identifiermeta.idchar