The Fermi-Lat CollaborationL. BaldiniJ. BalletD. BastieriJ. Becerra GonzalezR. BellazziniA. BerrettaE. BissaldiR. D. BlandfordE. D. BloomR. BoninoE. BottaciniP. BruelS. BusonR. A. CameronP. A. CaraveoE. CavazzutiS. ChenG. ChiaroD. CiangottiniN. CibarioS. CipriniP. Cristarella OrestanoM. CrnogorcevicS. CutiniF. D'AmmandoP. De La Torre LuqueF. De PalmaS. W. DigelN. Di LallaF. DirirsaL. Di VenereA. DomínguezA. FioriH. FleischhackA. FranckowiakY. FukazawaS. FunkP. FuscoF. GarganoD. GasparriniS. GermaniN. GigliettoF. GiordanoM. GirolettiD. GreenI. A. GrenierS. GriffinS. GuiriecM. GustafssonJ. W. HewittD. HoranR. ImazawaG. JóhannessonM. KerrD. KocevskiM. KussS. LarssonL. LatronicoJ. LiI. LiodakisF. LongoF. LoparcoM. N. LovelletteP. LubranoS. MalderaA. ManfredaG. Martí-DevesaH. MatakeM. N. MazziottaI. MereuM. MeyerN. MirabalW. MitthumsiriT. MizunoM. E. MonzaniA. MorselliI. V. MoskalenkoS. NagasawaM. NegroR. OjhaM. OrientiE. OrlandoM. PalatielloV. PaliyaD. PanequeZ. PeiM. PersicM. Pesce-RollinsV. PetrosianH. PoonT. A. PorterG. PrincipeJ. L. RacusinS. RainR. RandoB. RaniM. RazzanoS. RazzaqueA. ReimerOskar Klein CentreIstituto Nazionale di Fisica Nucleare, Sezione di TriesteIstituto Nazionale di Fisica Nucleare, Sezione di PerugiaIstituto Nazionale di Fisica Nucleare, Sezione di LecceIstituto Nazionale di Fisica Nucleare, Sezione di BariIstituto Nazionale di Fisica Nucleare, Sezione di Roma Tor VergataIstituto Nazionale di Fisica Nucleare, Sezione di TorinoIstituto Nazionale di Fisica Nucleare, Sezione di PisaIstituto Nazionale Di Fisica Nucleare, Sezione di PadovaUniversite Paris-SaclayAryabhatta Research Institute of Observational SciencesINAF Istituto di Astrofisica Spaziale e Fisica Cosmica, MilanKorea Astronomy and Space Science InstituteRaunvísindastofnun HáskólansAgenzia Spaziale ItalianaGeorg-August-Universität GöttingenHiroshima UniversityDeutsches Elektronen-Synchrotron (DESY)Osservatorio Astronomico di TriesteUniversità di PisaPolitecnico di BariUniversidad Complutense de MadridNaval Research LaboratoryUniversità degli Studi di UdineSLAC National Accelerator LaboratoryThe University of TokyoUniversity of Maryland, Baltimore County (UMBC)Universita del SalentoCatholic University of AmericaAmerican UniversityUniversity of Maryland, College ParkUniversité Savoie Mont BlancUniversità degli Studi di TriesteHögskolan DalarnaÉcole polytechniqueMahidol UniversityJulius-Maximilians-Universität WürzburgIstituto Di Radioastronomia, BolognaUniversità degli Studi di TorinoNASA Marshall Space Flight CenterInstituto Astrofisico de CanariasUniversität InnsbruckMax Planck Institute for Physics (Werner Heisenberg Institute)Turun yliopistoNASA Goddard Space Flight CenterRuhr-Universitat BochumThe George Washington UniversityUniversità degli Studi di PerugiaThe Royal Institute of Technology (KTH)Friedrich-Alexander-Universität Erlangen-NürnbergUniversity of JohannesburgUniversità degli Studi di PadovaUniversity of North Florida2022-08-042022-08-042021-09-01Astrophysical Journal, Supplement Series. Vol.256, No.1 (2021)006700492-s2.0-85115816543https://repository.li.mahidol.ac.th/handle/20.500.14594/76850We present the first Fermi Large Area Telescope (LAT) catalog of long-term γ-ray transient sources (1FLT). This comprises sources that were detected on monthly time intervals during the first decade of Fermi-LAT operations. The monthly timescale allows us to identify transient and variable sources that were not yet reported in other Fermi-LAT catalogs. The monthly data sets were analyzed using a wavelet-based source detection algorithm that provided the candidate new transient sources. The search was limited to the extragalactic regions of the sky to avoid the dominance of the Galactic diffuse emission at low Galactic latitudes. The transient candidates were then analyzed using the standard Fermi-LAT maximum likelihood analysis method. All sources detected with a statistical significance above 4σ in at least one monthly bin were listed in the final catalog. The 1FLT catalog contains 142 transient γ-ray sources that are not included in the 4FGL-DR2 catalog. Many of these sources (102) have been confidently associated with active galactic nuclei (AGNs): 24 are associated with flat-spectrum radio quasars, 1 with a BL Lac object, 70 with blazars of uncertain type, 3 with radio galaxies, 1 with a compact steep-spectrum radio source, 1 with a steep-spectrum radio quasar, and 2 with AGNs of other types. The remaining 40 sources have no candidate counterparts at other wavelengths. The median γ-ray spectral index of the 1FLT-AGN sources is softer than that reported in the latest Fermi-LAT AGN general catalog. This result is consistent with the hypothesis that detection of the softest γ-ray emitters is less efficient when the data are integrated over year-long intervals.Mahidol UniversityEarth and Planetary SciencesPhysics and AstronomyArticleSCOPUS10.3847/1538-4365/ac072a