Fermi Large Area Telescope Third Source Catalog (1501.02003v3)

Abstract: We present the third Fermi Large Area Telescope source catalog (3FGL) of sources in the 100 MeV-300 GeV range. Based on the first four years of science data from the Fermi Gamma-ray Space Telescope mission, it is the deepest yet in this energy range. Relative to the 2FGL catalog, the 3FGL catalog incorporates twice as much data as well as a number of analysis improvements, including improved calibrations at the event reconstruction level, an updated model for Galactic diffuse gamma-ray emission, a refined procedure for source detection, and improved methods for associating LAT sources with potential counterparts at other wavelengths. The 3FGL catalog includes 3033 sources above 4 sigma significance, with source location regions, spectral properties, and monthly light curves for each. Of these, 78 are flagged as potentially being due to imperfections in the model for Galactic diffuse emission. Twenty-five sources are modeled explicitly as spatially extended, and overall 232 sources are considered as identified based on angular extent or correlated variability (periodic or otherwise) observed at other wavelengths. For 1009 sources we have not found plausible counterparts at other wavelengths. More than 1100 of the identified or associated sources are active galaxies of the blazar class; several other classes of non-blazar active galaxies are also represented in the 3FGL. Pulsars represent the largest Galactic source class. From source counts of Galactic sources we estimate the contribution of unresolved sources to the Galactic diffuse emission is ~3% at 1 GeV.

• The paper presents the 3FGL catalog, reporting 3033 gamma-ray sources detected at a significance above 4σ over four years of observations.
• The paper uses enhanced calibration, refined source detection, and improved Galactic diffuse emission modeling to accurately associate gamma-ray sources with multiwavelength counterparts.
• The paper advances our understanding of high-energy astrophysics by providing observational constraints that refine models of cosmic ray interactions and galactic dynamics.

Fermi Large Area Telescope Third Source Catalog

The publication of the Fermi Large Area Telescope Third Source Catalog (3FGL) marks an essential contribution to the understanding of high-energy astrophysical phenomena. The catalog provides a comprehensive inventory of detected gamma-ray sources observed during the first four years of the Fermi Gamma-ray Space Telescope mission, covering the energy range from 100 MeV to 300 GeV. This release succeeds the LAT Bright Source List (0FGL) and the subsequent 1FGL and 2FGL catalogs, expanding upon their data by incorporating twice as much observational data as the 2FGL.

Key Findings and Data Inclusions

The 3FGL catalog comprises 3033 sources exceeding a detection significance threshold of 4σ. The catalog is the deepest LAT release in its energy range, owing to methodological improvements such as enhanced calibration at the event reconstruction level, an updated model for Galactic diffuse gamma-ray emission, refined source detection procedures, and better methods for associating LAT sources with potential counterparts at other wavelengths. The catalog reports 78 sources flagged due to potential imperfections in the Galactic diffuse emission model. In particular, the modeling of galactic interstellar emission shows remarkable fidelity, but certain regions, notably those around star-forming regions, still present challenges due to residual background emissions.

Source Classes and Associations

The catalog includes several notable astrophysical source classes:

• Galactic Sources: The catalog identifies 143 pulsars by their pulsations, encapsulating the largest class of identified Galactic sources. Twelve supernova remnants (SNRs) are identified through spatial morphology and are represented as extended sources.
• Extragalactic Sources: More than 1100 identified or associated sources are active galaxies of the blazar class. Additionally, several non-blazar active galaxies are cataloged, including some radio galaxies and Seyfert galaxies. The number of associations demonstrates the catalog's robust capability to connect gamma-ray sources with known extragalactic objects.

The identification of sources leverages correlations in angular magnitudes with observations across different wavelengths and periodic variability, particularly in pulsars. Consequently, the association and identification processes are highly reliable, offering insights into both firmly identified sources and those with significant associations.

Implications and Future Directions

The inclusion of extended sources and better cross-referenced associations at multiple wavelengths enriches the scientific usability of the catalog. Although primarily focused on identification and classification, the catalog suggests implications for understanding the spatial distribution of gamma-ray sources, contributing to models of cosmic ray interactions and galactic dynamics. For instance, it provides observational constraints useful for refining models of diffuse gamma-ray emission in the Milky Way.

Looking forward, the catalog sets the stage for deepening our understanding of gamma-ray sources and improving detection methods. The vast data collected may inform the development of more sophisticated modeling techniques, particularly for complex regions with bright diffuse emission backgrounds. Through the insights gleaned from the catalog, continued advancements in gamma-ray astronomy are anticipated, further unveiling the cosmic phenomena throughout our galaxy and beyond.

The increased depth and breadth of observations encapsulated in the 3FGL make it a pivotal resource for astrophysicists intending to explore high-energy sources, analyze their spectra, and probe the broader dynamics of our universe through the lens of gamma-ray emission.