Catalog of Long-Term Transient Sources in the First 10 Years of Fermi-LAT Data

Abstract

The catalog of long-term transient sources comprises sources that were detected on monthly time intervals during the first decade of Fermi-LAT operations (1FLT). The monthly time scale allows us to identify transient and variable sources that may have not been reported in Fermi-LAT general catalogs. The analysis was performed for photon energies between 0.1 and 300 GeV using the Pass-8 event-level selection. In the analysis we considered only photons with |b| > 10° to exclude the Galactic plane and therefore to avoid confusion with low-latitude diffuse emission. We have analyzed 120 months and also performed a 15-day shift of each month in order to not lose any flare at the edges of each time bin. The monthly datasets were analyzed using a wavelet-based source detection algorithm that provided the candidate new transient sources. The transient candidates were then analyzed using the standard Fermi-LAT maximum likelihood analysis method. The resulting catalog list has 142 different sources not included in 4FGL-DR2 and detected with a statistical significance above 4σ in at least one monthly bin. About 70% are associated with spectrally soft AGN-type counterparts, principally blazar candidates of uncertain type and flat-spectrum radio quasars, and about 30% of 1FLT sources remain unassociated. This is similar to the fraction of unassociated sources found in the Fermi-LAT general catalogs. The median γ-ray spectral index of the 1FLT-AGN sources is softer than the median index reported in the latest Fermi-LAT AGN general catalog (4LAC). In the 1FLT catalog are reported 6 sources listed also in a previous general catalog (1-3FGL).