juliensimon/fermi-4lac-agn-catalog

---
license: cc-by-4.0
pretty_name: "Fermi LAT Fourth AGN Catalog (4LAC)"
language:
- en
description: "Active galactic nuclei detected by the Fermi Large Area Telescope, the largest gamma-ray AGN catalog with source classifications, spectral parameters, and redshifts."
task_categories:
- tabular-classification
tags:
- space
- gamma-ray
- fermi
- nasa
- agn
- blazars
- astronomy
- open-data
- tabular-data
size_categories:
- 1K<n<10K
configs:
- config_name: default
data_files:
- split: train
path: data/fermi-4lac.parquet
default: true
---

# Fermi LAT Fourth AGN Catalog (4LAC)

*Part of the [Astronomy Datasets](https://huggingface.co/collections/juliensimon/astronomy-datasets-69c24caf2f17e36128946743) collection on Hugging Face.*

The largest catalog of gamma-ray active galactic nuclei (AGN), detected by the
[Fermi Large Area Telescope (LAT)](https://fermi.gsfc.nasa.gov/). 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.

## Schema

| Column | Type |
|--------|------|
| `__row` | string |
| `name` | string |
| `data_release` | string |
| `ra` | float |
| `dec` | float |
| `lii` | float |
| `bii` | float |
| `detection_significance` | string |
| `flux_1_100_gev` | string |
| `flux_1_100_gev_error` | string |
| `energy_flux` | float |
| `energy_flux_error` | string |
| `spectrum_type` | string |
| `pl_index` | string |
| `pl_index_error` | string |
| `pivot_energy` | float |
| `lp_index` | float |
| `lp_index_error` | string |
| `lp_beta` | float |
| `lp_beta_error` | string |
| `analysis_flags` | string |
| `source_type` | string |
| `assoc_name` | string |
| `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 |
| `sed_class` | string |
| `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 |
| `frac_variability` | float |
| `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

```python
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](https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermilac.html)
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](https://huggingface.co/datasets/juliensimon/gamma-ray-bursts) -- Fermi GBM Gamma-Ray Burst Catalog
- [pulsar-catalog](https://huggingface.co/datasets/juliensimon/pulsar-catalog) -- ATNF Pulsar Catalogue
- [near-earth-objects](https://huggingface.co/datasets/juliensimon/near-earth-objects) -- NEO close approaches

## Pipeline

Source code: [juliensimon/space-datasets](https://github.com/juliensimon/space-datasets)

## Citation

```bibtex
@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](https://creativecommons.org/licenses/by/4.0/)