Incremental Fermi Large Area Telescope Fourth Source Catalog

Abstract: We present an incremental version (4FGL-DR3, for Data Release 3) of the fourth Fermi-LAT catalog of gamma-ray sources. Based on the first twelve years of science data in the energy range from 50 MeV to 1 TeV, it contains 6658 sources. The analysis improves on that used for the 4FGL catalog over eight years of data: more sources are fit with curved spectra, we introduce a more robust spectral parameterization for pulsars, and we extend the spectral points to 1 TeV. The spectral parameters, spectral energy distributions, and associations are updated for all sources. Light curves are rebuilt for all sources with 1 yr intervals (not 2 month intervals). Among the 5064 original 4FGL sources, 16 were deleted, 112 are formally below the detection threshold over 12 yr (but are kept in the list), while 74 are newly associated, 10 have an improved association, and seven associations were withdrawn. Pulsars are split explicitly between young and millisecond pulsars. Pulsars and binaries newly detected in LAT sources, as well as more than 100 newly classified blazars, are reported. We add three extended sources and 1607 new point sources, mostly just above the detection threshold, among which eight are considered identified, and 699 have a plausible counterpart at other wavelengths. We discuss degree-scale residuals to the global sky model and clusters of soft unassociated point sources close to the Galactic plane, which are possibly related to limitations of the interstellar emission model and missing extended sources.

• The paper presents an updated catalog from 12 years of Fermi LAT data, substantially improving source detection and spectral characterization.
• The paper employs refined spectral models and advanced localization techniques, identifying 1607 new point sources with over 50% of sources showing curved spectra.
• The paper highlights a significant fraction of unassociated sources near the Galactic plane, offering insights into potential pulsars and unmodeled diffuse emissions.

Incremental Fermi Large Area Telescope Fourth Source Catalog

The presented paper details the Incremental Fermi Large Area Telescope Fourth Source Catalog (4FGL-DR3), which embodies an augmented collection based on 12 years of observational data ranging from 50 MeV to 1 TeV. This data set has yielded 6658 sources. The introduction of broader spectral modeling, enhanced association accuracies, and incremental updates mark this compilation as a notable evolution from its previous versions.

Data and Methodology

The 4FGL-DR3 uses data collected from August 2008 to August 2020. It utilizes the Pass 8 event reconstruction, which enhances the sensitivity and accuracy of the Fermi Large Area Telescope (LAT). The extended catalog incorporates updated calibrations and a revised diffuse emission model, though the underlying methodology remains largely faithful to analytical practices established by 4FGL. Among the significant modifications is a refined approach to the spectral characterization of sources, tailored to improve completeness and accuracy, particularly in the low- and high-energy extremes of the observational spectrum.

Structural Updates and Analytical Innovations

The catalogue introduces noteworthy updates. Expanded spectral coverage now includes a substantial extension to higher energies, with sources now fit with curved spectral models when possible. This adjustment lessens the prevailing biases of previous flat-spectrum assumptions, especially in low-TS sources. The incorporation of new methods for localizing sources and assessing systematic uncertainties refines position accuracies, fostering improved reliability of source identifications against known counterparts across diverse catalogs.

Numerical Highlights

Significant numerical outputs include 1607 newly identified point sources, with expanded coverage including at least 100 additional blazars compared to previous releases. Furthermore, over 50% of all sources now exhibit spectrally curved models, reflecting improved statistical descriptors applied across the data set.

Unassociated Sources and Classifications

An outstanding element of the DR3 release is the large fraction of unassociated sources; these represent more than 30% of the catalog. Particularly clustered around the Galactic plane, these unassociated sources pose intriguing questions regarding their collective origin and prompt discussion on possible connections to presently unmodeled diffuse emissions. There is potential for these sources to harbor yet-undetected pulsars or other compact objects, as well as insights into limitations of current interstellar emission models.

Implications and Future Considerations

The development and deployment of the Fermi 4FGL-DR3 provides crucial insights into high-energy cosmic gamma radiation, granting broader understanding and generating new avenues of astrophysical inquiry. By maintaining an incremental approach, this catalog aims to preserve continuity and comparability with previous data while gradually introducing methodological enhancements. As the Fermi mission evolves, anticipating enhancements in data analysis techniques, including those encompassing machine learning and AI-driven methodologies, is essential. These advances will further refine the catalog, potentially unlocking novel insights into both identified and unassociated celestial sources.

In conclusion, the 4FGL-DR3 catalog epitomizes a strategic step forward in high-energy astrophysics. It acknowledges the challenges tied to accurately characterizing numerous faint and distant gamma-ray sources while setting the groundwork for future enhancements in gamma-ray astronomy. This persistent effort to accurately catalogue the gamma-ray sky supports a framework through which the broader scientific community can explore and unravel some of the highest energy phenomena in the universe.