Incremental Fermi Large Area Telescope Fourth Source Catalog

Abstract

We present an incremental version (4FGL-DR3, for Data Release 3) of the fourth Fermi Large Area Telescope (LAT) catalog of γ-ray sources. Based on the first 12 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(but are kept in the list), while 74 are newly associated, 10 have an improved association, and seven associations were withdrawn.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 the 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.

Serendipitous Discovery of Three Millisecond Pulsars with the GMRT in Fermi-directed Survey and Follow-up Radio Timing

“…In this work we present the discovery of three new MSPs in this GMRT survey. Even though all three MSPs were discovered in error circles of LAT sources from the 1FGL catalog (Abdo et al 2010), none have associated sources in the 4FGL−DR3 catalog (an incremental update to the 4FGL catalog using 12 yr of LAT data; Abdollahi et al 2020;Abollahi et al 2022). This suggests that the MSPs are likely unassociated with the original Fermi γ-ray point sources targeted in the survey, i.e., they are serendipitious discoveries.…”

The Isotropic γ-ray Emission above 100 GeV: Where Do Very High-energy γ-rays Come From?

“…For energies > 100 GeV and Galactic latitudes b > |50°|, we would expect the extragalactic isotropic γ-ray emission to be dominated by blazars or other potential VHE sources, like starforming and radio galaxies. It is unreasonable to assume that these VHE equivalent photons originate from Galactic VHE objects, like pulsar wind nebulae and supernova remnants, as out of 62 such objects listed in 4FGL-DR3, only two have b ∼ 32°(both in the Large Magellanic Cloud), and the rest have b < |10°| (Abdollahi et al 2020(Abdollahi et al , 2022. Above |50°| in Galactic latitude, the majority of VHE γ-ray emitters must be extragalactic.…”

“…The large positional uncertainty of γ-ray observations, especially below 1 GeV, makes the association of γ-ray sources with their low-energy counterparts a challenging task (Massaro et al 2015bLandoni et al 2015;Ricci et al 2015;de Menezes et al 2020a) as several astrophysical objects can lie within their positional reconstruction confidence regions Abdollahi et al 2020Abdollahi et al , 2022. Therefore, the astrophysical community relies on statistical association methods, where observed quantities are used to compute an association probability between the γ-ray source and its counterpart candidates (Sutherland & Saunders 1992;Ackermann et al 2011;Abdollahi et al 2020;de Menezes et al 2020b).…”

“…At such energies, the 68% containment radius for a single photon detection with Fermi-LAT is ∼0°.1 (Atwood et al 2009), and therefore this single detection could unveil the presence of a blazar. Blazars indeed account for nearly 80% of the sources listed in the third data release of the Fermi-LAT fourth source catalog (4FGL-DR3; Abdollahi et al 2020Abdollahi et al , 2022 for such Galactic latitudes and are prone to emit VHE photons via inverse Compton processes (especially BL Lac objects).…”

easyFermi: a graphical interface for performing Fermi-LAT data analyses

“…Once the events detected by the Fermi -LAT are classified, the collected γ-ray data are released to the public on the Fermi Science Support Center (FSSC) data server 2 . The users of these data can rely on Fermitools 3 and Fermipy 4 [11] to perform their analyses, both tools relying on several online tutorials 5 .…”