juliensimon/fermi-4lac-agn-catalog

Fermi LAT Fourth AGN Catalog (4LAC)

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

Part of a dataset collection on Hugging Face.

Dataset description

Active galactic nuclei (AGN) detected by the Fermi Large Area Telescope, the largest gamma-ray AGN catalog with source classifications, spectral parameters, and redshifts.

Active galactic nuclei 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 due to dominant external Compton scattering. 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.

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, constraining physical jet models and distinguishing between leptonic and hadronic emission scenarios.

This dataset is suitable for tabular classification tasks.

Schema

Column	Type	Description	Sample	Null %
name	str	IAU source name (4FGL designation)	4FGL J2109.8-8618	0.0%
ra	float64	Right ascension (J2000, degrees, 0-360)	317.4619	0.0%
dec	float64	Declination (J2000, degrees, -90 to +90)	-86.3078	0.0%
lii	float64	Galactic longitude (degrees, 0-360)	306.4125549	0.0%
bii	float64	Galactic latitude (degrees, -90 to +90)	-29.18188524	0.0%
energy_flux	float64	Energy flux in erg/cm2/s integrated over 100 MeV-100 GeV	3.1895e-12	0.0%
pivot_energy	float64	Decorrelation (pivot) energy in MeV at which the flux uncertainty is minimized; minimizes spectral index / normalization degeneracy	6264.75	0.0%
lp_index	float64	Log-parabola spectral index at the pivot energy	1.7607	0.0%
lp_beta	float64	Log-parabola curvature parameter; >0 indicates spectral softening at higher energies	0.0366	0.0%
assoc_name	str	Counterpart name at other wavelengths (radio, optical, or X-ray); null if no confident association	2MASS J21101113-8618473	0.0%
redshift	float64	Spectroscopic redshift; null for ~40% of BL Lac objects that lack optical emission lines	1.032	47.0%
sed_class	str	Synchrotron peak classification: LSP (low-synchrotron-peaked), ISP (intermediate), HSP (high)	HSP	22.8%
variability_index	float64	Flux variability chi-squared statistic over the 4-year baseline; >18.48 indicates significant variability at 99% confidence	10.4192	0.0%
frac_variability	float64	Fractional variability amplitude F_var; null if source is not significantly variable	0.0	0.0%

Quick stats

• 3,407 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()

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"]]
print(top)

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

Data source

https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermilac.html

Related datasets

• juliensimon/gamma-ray-bursts

• juliensimon/pulsar-catalog

• juliensimon/fermi-4fgl-dr4

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About the author

Created by Julien Simon — AI Operating Partner at Fortino Capital. Part of the Space Datasets collection.

Citation

@dataset{fermi_4lac_agn_catalog,
  title = {Fermi LAT Fourth AGN Catalog (4LAC)},
  author = {juliensimon},
  year = {2026},
  url = {https://huggingface.co/datasets/juliensimon/fermi-4lac-agn-catalog},
  publisher = {Hugging Face}
}

License

CC-BY-4.0