Steps toward the 5FGL Fermi-LAT source catalog

The current Fermi-LAT source catalog (4FGL-DR4: 7194 sources over 14 years) was built incrementally from the 8-year catalog. In a survey mission like Fermi, data accumulate on each source over time, so after 16 years (reached in August 2024) and twice the data for the original 4FGL sources we have much more precise localization. It is then time to generate a new original catalog (5FGL), which implies, beyond adding the sources newly detectable after two more years, changing the existing source names (derived from their coordinates) and reviewing the associations. The systematic errors due to our imperfect knowledge of the Galactic diffuse emission, which dominates the gamma-ray sky, are the major limiting factor of the sensitivity and quality of the point source catalog at low energies (a few 100 MeV over the extragalactic sky, up to a few GeV in the Galactic ridge). Two parallel efforts are ongoing to improve on this situation, using more advanced methods to modulate the interstellar gas templates with the LAT data themselves. In parallel an early 16-year list (FL16Y) of 7217 sources is presented, which relocalizes all sources and improves a few aspects of the catalog analysis, but still uses the same diffuse model as 4FGL-DR4.