[Paper Review] Fermi Large Area Telescope Fourth Source Catalog

LAT Collaboration|arXiv (Cornell University)|Feb 26, 2019

Astrophysics and Cosmic PhenomenaPhysics and Astronomy133 references120 citations

TL;DR

The paper presents the 4FGL catalog, the deepest LAT gamma-ray source catalog to date, built from eight years of Fermi-Lermi LAT data with updated diffuse modeling, energy dispersion correction, and light curves.

ABSTRACT

We present the fourth Fermi Large Area Telescope catalog (4FGL) of gamma-ray sources. Based on the first eight years of science data from the Fermi Gamma-ray Space Telescope mission in the energy range from 50 MeV to 1 TeV, it is the deepest yet in this energy range. Relative to the 3FGL catalog, the 4FGL catalog has twice as much exposure as well as a number of analysis improvements, including an updated model for the Galactic diffuse gamma-ray emission, and two sets of light curves (1-year and 2-month intervals). The 4FGL catalog includes 5064 sources above 4 sigma significance, for which we provide localization and spectral properties. Seventy-five sources are modeled explicitly as spatially extended, and overall 358 sources are considered as identified based on angular extent, periodicity or correlated variability observed at other wavelengths. For 1336 sources we have not found plausible counterparts at other wavelengths. More than 3130 of the identified or associated sources are active galaxies of the blazar class, and 239 are pulsars.

Motivation & Objective

• Provide a comprehensive census of gamma-ray sources detected by Fermi-LAT over the first eight years of the mission.
• Improve source localization, spectra, and classification with Pass 8 data, updated diffuse models, and PSF-based event types.
• Characterize source types, associations, and identifications across wavelengths.
• Incorporate light curves and variability assessments over multiple time scales.
• Deliver an accessible catalog format and auxiliary materials for the community.

Proposed method

• Use eight years of LAT data (2008–2016) with Pass 8 P8R3_SOURCE_V2 event selection and PSF event types to optimize angular resolution.
• Split the data into six energy intervals and 15 components for summed likelihood analyses (MSL) across regions of interest.
• Apply a new diffuse Galactic emission model and include energy dispersion corrections where relevant.
• Test sources with three spectral models (power law, log normal, and power law with subexponential cutoff).
• Explicitly model 75 extended sources and build two sets of light curves (1-year and 2-month intervals).
• Update counterpart catalogs and perform association/identification procedures to link LAT sources to multiwavelength counterparts.

Experimental results

Research questions

• RQ1 What is the improved census of gamma-ray sources in the first eight years of LAT data compared to previous catalogs?
• RQ2 How do updated diffuse emission modeling and PSF-based event types affect source localization, spectral properties, and identification rates?
• RQ3 What is the distribution of source classes (e.g., AGN, pulsars) and the fraction with no plausible counterparts?
• RQ4 How do light curves and variability metrics in 1-year and 2-month bins inform source behavior and classification?
• RQ5 How does energy dispersion correction influence the gamma-ray spectra of detected sources?

Key findings

• The 4FGL catalog includes 5064 sources above 4 sigma significance.
• 75 sources are modeled explicitly as spatially extended.
• A total of 358 sources are identified based on angular extent, periodicity, or correlated multiwavelength variability.
• 1336 sources have no plausible counterparts at other wavelengths.
• More than 3130 identified or associated sources are blazars (AGN) and 239 are pulsars.
• The catalog uses eight years of data, doubled exposure relative to 3FGL, and includes light curves and updated diffuse modeling.