Catalog of Long-term Transient Sources in the First 10 yr of Fermi-LAT Data

Abstract

We 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 steepspectrum 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. Unified AstronomyThesaurus concepts: High energy astrophysics (739); Active galactic nuclei (16); Gamma-ray bursts (629); Solar flares (1496); Solar gamma-ray emission (1497); Gamma-rays (637); Gamma-ray sources (633); Catalogs (205); Gamma-ray telescopes (634); Gamma-ray transient sources (1853); Blazars (164); Quasars (1319) Supporting material: figure set, machine-readable tables, FITS file 74 Funded by contract FIRB-2012-RBFR12PM1F from the Italian Ministry of Education, University and Research (MIUR).

Incremental Fermi Large Area Telescope Fourth Source Catalog

“…The variable new sources are soft on average, similar to Flat Spectrum Radio Quasars. The same effect is observed in the 1FLT catalog (Baldini et al 2021), even more strongly because detections in 1FLT are based on onemonth data sets. The non-variable new sources have a peak on the soft side as well (probably similar to the variable ones, but not bright enough to be significantly variable).…”

“…Monthly light curves are now provided with similar accuracy in the Fermi -LAT Light Curve Repository 1 . Detection of transient sources on monthly time scales is covered by 1FLT 2 (Baldini et al 2021). Transients on weekly time scales are covered by FAVA 3 (Abdollahi et al 2017).…”

Gamma-ray Cosmology and Tests of Fundamental Physics

“…The dashed gray line shows significant detections by the EGRET and Fermi-LAT gammaray satellites at energies larger than 100 MeV. The pie charts display the distribution of sub-classes of extragalactic sources, as extracted from the 1EG [19], 3EG [10], 1FGL [17], 3FGL [20], 1FLGC [21], 4FGL-DR2 [22], 1FLT [23] and 2FLGC [24] catalogs. The solid black line shows significant detections by gamma-ray telescopes above 100 GeV as extracted from TeVCat [14] after the 2021 international cosmic-ray conference (ICRC 2021 stellar size and their outbursts are understood as being induced by the collapse of short-lived massive stars or by a binary merger.…”

Incremental Fermi Large Area Telescope Fourth Source Catalog

“…Monthly light curves, including indications of spectral variability, are now provided with similar accuracy in the Fermi-LAT Light Curve Repository. 85 Detection of transient sources on monthly timescales is covered by 1FLT 86 (Baldini et al 2021). Transients on weekly timescales are covered by FAVA 87 (Abdollahi et al 2017).…”