This paper presents the Fourth Catalog of Hard Fermi‑LAT Sources (4FHL), a new inventory of objects seen in very high‑energy gamma rays. The Fermi Large Area Telescope (LAT) team used 16 years of data to compile sources detected between 50 giga‑electronvolts (GeV) and 2 tera‑electronvolts (TeV). The catalog contains 673 objects and is the deepest all‑sky survey above 50 GeV to date. Compared with the previous LAT catalog that focused on the same energy band, the sensitivity and source positions are improved by roughly a factor of two.

To build the catalog the researchers started from Pass 8 Release 3 LAT data and selected events with reconstructed energies from 50 GeV to 2 TeV. They filtered out likely contamination from the edge of the Earth’s atmosphere and applied standard quality cuts. The final photon set used for the survey included about 133,400 events across the whole sky, roughly three photons per square degree. Candidate sources were found with multiple seed methods (sliding‑cell and wavelet searches, and by re‑examining known catalogs), producing over 11,000 initial candidates. The team then tested these candidates with an unbinned maximum‑likelihood analysis across 154 regions of interest, an approach that treats each detected photon individually and helps find faint sources when photon counts are low.

The catalog’s content is already informative. About 85 percent of the cataloged sources are associated with objects outside our Galaxy, mostly BL Lacertae objects — a class of active galaxies with jets aimed toward Earth. The catalog includes 36 extragalactic sources at redshift greater than 1. Galactic objects make up about 10 percent of the list, dominated by pulsar wind nebulae (clouds of energetic particles driven by a spinning neutron star) and supernova remnants. About five percent of the entries remain unassociated or uncertain, and 20 sources have no counterpart in earlier LAT catalogs.