# The Fourth Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope

## TL;DR

This paper presents the fourth catalog of active galactic nuclei detected by the Fermi Large Area Telescope, significantly expanding the known gamma-ray AGN population with detailed classifications and spectral properties.

## Contribution

It provides an updated, comprehensive catalog of 2863 gamma-ray AGNs, including new sources, classifications, and spectral characteristics, based on 8 years of Fermi data.

## Key findings

- 79% of high-latitude sources are AGNs
- Most AGNs are blazars, with 24% FSRQs and 38% BL Lacs
- FSRQs have softer spectra and higher variability

## Abstract

The fourth catalog of active galactic nuclei (AGNs) detected by the Fermi Gamma-ray Space Telescope Large Area Telescope (4LAC) between 2008 August 4 and 2016 August 2 contains 2863 objects located at high Galactic latitudes (|b|>10{\deg}). It includes 85% more sources than the previous 3LAC catalog based on 4 years of data. AGNs represent at least 79% of the high-latitude sources in the fourth Fermi-Large Area Telescope Source Catalog (4FGL), which covers the energy range from 50 MeV to 1 TeV. In addition, 344 gamma-ray AGNs are found at low Galactic latitudes. Most of the 4LAC AGNs are blazars (98%), while the remainder are other types of AGNs. The blazar population consists of 24% Flat Spectrum Radio Quasars (FSRQs), 38% BL Lac-type objects (BL Lacs), and 38% blazar candidates of unknown types (BCUs). On average, FSRQs display softer spectra and stronger variability in the gamma-ray band than BL Lacs do, confirming previous findings. All AGNs detected by ground-based atmospheric Cherenkov telescopes are also found in the 4LAC.