3FHL: The Third Catalog of Hard Fermi-LAT Sources

Abstract: We present a catalog of sources detected above 10 GeV by the Fermi Large Area Telescope (LAT) in the first 7 years of data using the Pass 8 event-level analysis. This is the Third Catalog of Hard Fermi-LAT Sources (3FHL), containing 1556 objects characterized in the 10 GeV - 2 TeV energy range. The sensitivity and angular resolution are improved by factors of 3 and 2 relative to the previous LAT catalog at the same energies (1FHL). The vast majority of detected sources (79%) are associated with extragalactic counterparts at other wavelengths, including 16 sources located at very high redshift ($z>2$). Eight percent of the sources have Galactic counterparts and 13% are unassociated (or associated with a source of unknown nature). The high-latitude sky and the Galactic plane are observed with a flux sensitivity of 4.4 to 9.5$\times 10^{-11}$ ph cm$^{-2}$ s$^{-1}$, respectively (this is approximately 0.5% and 1% of the Crab Nebula flux above 10 GeV). The catalog includes 214 new $\gamma$-ray sources. The substantial increase in the number of photons (more than 4 times relative to 1FHL and 10 times to 2FHL) also allows us to measure significant spectral curvature for 32 sources and find flux variability for 163 of them. Furthermore, we estimate that for the same flux limit of $10^{-12}$ erg cm$^{-2}$ s$^{-1}$, the energy range above 10 GeV has twice as many sources as above 50 GeV, highlighting the importance, for future Cherenkov telescopes, of lowering the energy threshold as much as possible.

• The paper presents the creation of the 3FHL catalog with 1556 sources, including 214 new gamma-ray detections from enhanced Fermi LAT observations.
• It utilizes 7 years of Pass 8 data to achieve improved sensitivity and angular resolution, enabling robust characterization of high-energy celestial sources.
• The findings reveal a predominance of extragalactic sources (79%), offering key insights into AGN variability, spectral curvature, and cosmic-ray propagation.

Overview of 3FHL: The Third Catalog of Hard Fermi Sources

The paper "3FHL: The Third Catalog of Hard Fermi Sources" presents an extensive survey conducted by the Fermi Large Area Telescope (LAT), focusing on the detection of sources above 10 GeV. This research builds upon previous Fermi LAT data and aims to provide a more comprehensive understanding of high-energy γ-ray sources. The study utilizes seven years of data obtained via the Pass 8 event-level analysis, which offers enhancements in sensitivity and angular resolution. This culminates in the creation of the 3FHL catalog, encompassing 1556 sources characterized within the 10 GeV to 2 TeV energy range, significantly improving upon the previous LAT catalog, 1FHL.

Key Findings

• Catalog Size and Growth: The 3FHL catalog introduces 214 new γ-ray sources, presenting a total of 1556 sources, which signifies a threefold increase in comparison to the 1FHL catalog that listed 514 sources. This expansion highlights the enhanced sensitivity achieved by the Pass 8 analysis, which facilitated the detection of weaker sources.
• Source Characterization: In terms of source categorization, 79% of the entries are linked with extragalactic counterparts, 8% with galactic origins, and 13% remain unassociated or of unknown classification. This suggests that the majority of high-energy sources are extragalactic in nature, predominantly active galactic nuclei (AGN) including blazars, with a minority related to Galactic sources such as pulsars and supernova remnants (SNRs).
• Flux Sensitivity and Energy Distribution: The 3FHL offers a significant enhancement in flux sensitivity and angular resolution (by factors of approximately 3 and 2 respectively) relative to its predecessor at comparable energies. The catalog documents a flux sensitivity of 4.4 to 9.5 x 10^-11 ph cm^-2 s^-1, which highlights the improved detection capabilities facilitated by Pass 8.
• Spectral and Variability Analysis: Analysis of spectral data allowed for the identification of spectral curvature in 32 sources and flux variability in 163 sources, pointing to complex physical processes in these systems. Spectral indices for the sources cluster around a median value of 2.48, indicating relatively hard spectra.

Implications and Future Directions

By extending the localization and sensitivity of Fermi LAT data in the high-energy regime, 3FHL serves as a valuable resource for future γ-ray observatories, especially those planning to expand operations into the lower energy thresholds currently surveyed by ground-based Cherenkov telescopes. The overlap between space-based and ground-based observations will vitalize comparative studies and provide a coherent view of γ-ray skies, critical for understanding both galactic and extragalactic high-energy phenomena.

The potential for this expanded dataset to enhance understanding of unresolved cosmic-ray interactions, dark matter searches, and the Extragalactic Background Light (EBL) is significant. The data's interplay with theoretical models presents opportunities for corroborative studies in both astrophysics and particle physics, with particular relevance to studies of AGN variability and cosmic-ray propagation.

Conclusion

The paper underscores the advancements made in the detection and characterization of high-energy sources using Fermi LAT, which are articulated through the cataloging of a comprehensive dataset in 3FHL. The implications of this expanded catalog for future observational platforms, theoretical modeling, and the broader understanding of the universe's high-energy processes are substantial, establishing a foundation for ongoing exploration of the γ-ray cosmos.