juliensimon/fermi-4lac-agn-catalog

Fermi LAT Fourth AGN Catalog (4LAC)

Credit: NASA/JPL-Caltech/STScI/CXC/SAO

Part of the Astronomy Datasets collection on Hugging Face.

The largest catalog of gamma-ray active galactic nuclei (AGN), detected by the Fermi Large Area Telescope (LAT). Currently 3,409 sources with classifications, spectral parameters, and multiwavelength associations.

Dataset description

Active galactic nuclei (AGN) are supermassive black holes at the centers of galaxies that produce powerful jets of relativistic particles. When one of these jets points toward Earth, the AGN appears as a blazar -- the most common type of gamma-ray source in the sky.

The Fourth LAT AGN Catalog (4LAC) is based on Fermi LAT observations and represents the most comprehensive census of gamma-ray AGN. It includes BL Lac objects, flat-spectrum radio quasars (FSRQs), and other AGN types, with spectral parameters, variability indices, and multiwavelength counterpart associations.

The two dominant blazar subclasses — BL Lac objects and FSRQs — represent fundamentally different accretion regimes onto supermassive black holes. FSRQs are high-luminosity sources with strong broad emission lines, radiatively efficient accretion disks, and gamma-ray spectra that tend to be soft (steep spectral indices) due to dominant external Compton scattering off photons from the broad-line region or dusty torus. BL Lac objects have weak or absent emission lines, radiatively inefficient accretion flows, and harder gamma-ray spectra produced primarily by synchrotron self-Compton emission within the jet. This spectral dichotomy underpins the "blazar sequence" — the observed anti-correlation between bolometric luminosity and the peak frequency of the synchrotron spectral component — though its physical origin remains debated.

The 4LAC is a cornerstone for AGN unification studies and jet physics. The redshift distribution encodes the cosmological evolution of the blazar population: FSRQs show strong positive evolution (more numerous and luminous at higher redshifts, peaking around z ~ 1-2), tracing the epoch of peak supermassive black hole growth, while BL Lac objects show weaker or negative evolution. The spectral parameters — power-law indices, log-parabola curvatures, and pivot energies — constrain particle acceleration mechanisms and the jet magnetic field structure. Variability indices identify flaring sources that are prime targets for very-long-baseline interferometry (VLBI) imaging of superluminal jet components and for multi-messenger searches in neutrino data.

The gamma-ray properties in 4LAC, combined with radio, optical, and X-ray data, enable construction of broadband spectral energy distributions (SEDs) spanning over 15 decades in frequency. These SEDs are the primary observational tool for constraining physical jet models — distinguishing between leptonic scenarios (where electrons and positrons radiate via synchrotron and inverse-Compton processes) and hadronic models (where protons accelerated in the jet produce gamma rays through pion cascades, with accompanying neutrino emission).

Schema

Column	Type	Description
__row	string
name	string	IAU source name (4FGL designation)
data_release	string
ra	float	Right ascension (J2000, degrees)
dec	float	Declination (J2000, degrees)
lii	float	Galactic longitude (degrees)
bii	float	Galactic latitude (degrees)
detection_significance	string
flux_1_100_gev	string
flux_1_100_gev_error	string
energy_flux	float	Energy flux (erg/cm2/s, 100 MeV-100 GeV)
energy_flux_error	string
spectrum_type	string
pl_index	string
pl_index_error	string
pivot_energy	float	Pivot energy for spectral fit (MeV)
lp_index	float	Log-parabola spectral index at pivot energy
lp_index_error	string
lp_beta	float	Log-parabola curvature parameter
lp_beta_error	string
analysis_flags	string
source_type	string
assoc_name	string	Associated source name from counterpart catalog
assoc_prob_bay	string
assoc_prob_lr	string
assoc_catalog	string
assoc_ra	string
assoc_dec	string
assoc_error_radius	string
vlbi_assoc_name	string
redshift	float	Source redshift (spectroscopic or photometric)
sed_class	string	SED-based classification (LSP, ISP, HSP)
he_peak	string
he_peak_error	string
he_nufnu_peak	string
he_nufnu_peak_error	string
nu_syn	string
nufnu_syn	string
variability_index	float	Variability index from likelihood analysis
frac_variability	float	Fractional variability amplitude
frac_variability_error	string
highest_energy_photon	string
__x_ra_dec	string
__y_ra_dec	string
__z_ra_dec	string

Quick stats

• 3,409 active galactic nuclei
• 1,806 sources with measured redshift
• Median redshift: 0.560

Usage

from datasets import load_dataset

ds = load_dataset("juliensimon/fermi-4lac-agn-catalog", split="train")
df = ds.to_pandas()

# Brightest AGN by flux
top = df.nlargest(10, "flux")[["name", "flux", "spectral_index", "redshift"]] if "name" in df.columns else df.nlargest(10, "flux")

# Redshift distribution
import matplotlib.pyplot as plt
df["redshift"].dropna().hist(bins=50)
plt.xlabel("Redshift")
plt.title("4LAC AGN Redshift Distribution")

Data source

All data comes from the Fermi LAT 4LAC Catalog hosted by NASA's High Energy Astrophysics Science Archive Research Center (HEASARC), accessed via the TAP protocol.

Reference: Ajello, M. et al. (2020), "The Fourth Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope", ApJ, 892, 105.

Related datasets

• gamma-ray-bursts -- Fermi GBM Gamma-Ray Burst Catalog
• pulsar-catalog -- ATNF Pulsar Catalogue
• near-earth-objects -- NEO close approaches

Pipeline

Source code: juliensimon/space-datasets

Support

If you find this dataset useful, please give it a ❤️ on the dataset page and share feedback in the Community tab! Also consider giving a ⭐️ to the space-datasets repo.

Citation

@dataset{fermi_4lac,
  author = {Simon, Julien},
  title = {Fermi LAT Fourth AGN Catalog (4LAC)},
  year = {2026},
  publisher = {Hugging Face},
  url = {https://huggingface.co/datasets/juliensimon/fermi-4lac-agn-catalog},
  note = {Based on Fermi LAT 4LAC (Ajello et al. 2020) via NASA HEASARC}
}

License

CC-BY-4.0