A Catalogue of Variable Active Galactic Nuclei Based on Multi-Timescale Variability Analysis from Fermi-LAT Data

Active Galactic Nuclei (AGN) sources feature supermassive black holes that launch relativistic plasma jets. They are key $γ$-ray sources providing a unique laboratory for studying extreme particle acceleration and plasma physics. Variability in $γ$-ray emission is an important signature that may constrain the size of the emission region and the physical processes driving flares. However, current large-scale $γ$-ray catalogs, such as the Fermi-LAT 4LAC-DR3, typically characterize variability only on long timescales (yearly or 60-day), lacking necessary constraints on short-term behavior from days to weeks. To address this, we systematically characterize $γ$-ray variability in AGNs across short timescales: 3-day, 7-day (weekly), and 30-day (monthly). We present a preliminary catalogue of variable AGN based on light curves from the Fermi-LAT Light Curve Repository. We show that the variability amplitude ($σ_{\rm NXS}^{2}$) presents similar values across different timescales, potentially increasing for a subsample of sources as the observation timescale increases. This high-cadence analysis reinforces the known dichotomy between flat-spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs), with FSRQs consistently exhibiting stronger variability. By identifying the most luminous and variable sources at each timescale, we highlight key targets for follow-up with next-generation observatories such as the Cherenkov Telescope Array Observatory (CTAO), ASTRI Mini-Array, and the Southern Wide-field Gamma-ray Observatory (SWGO), where strong short-term variability suggests highly compact emission zones and extreme particle acceleration efficiency. This catalogue contributes to the understanding of high-energy outflows in AGN jets and provides a foundation for optimizing observational strategies through a unified variability metric across timescales.

💡 Research Summary

The paper presents a systematic, high‑cadence variability study of active galactic nuclei (AGN) using γ‑ray light curves from the Fermi‑LAT Light Curve Repository. The authors selected 1 429 AGN with available 3‑day, 7‑day (weekly), and 30‑day (monthly) binned flux measurements, applied quality cuts including an outlier treatment and a test‑statistic threshold of TS > 4, and computed the normalized excess variance (NXS, σ²_NXS) as the primary variability metric. NXS is defined as the variance of the fluxes normalized by the squared mean flux, corrected for measurement errors, and the fractional variability F_var = max(0, √σ²_NXS) is used to ensure statistical robustness.

The analysis shows that σ²_NXS values are broadly similar across the three timescales, with most sources clustering around the 1:1 line when shorter‑timescale NXS is plotted against longer‑timescale NXS. A systematic tail of points lies below the 1:1 line, indicating that for a subset of AGN the variability amplitude modestly increases with the length of the observation window (σ²_NXS,short ≤ σ²_NXS,long). This behavior is visualized in Figure 1, a 3 × 3 matrix where rows correspond to spectral‑energy‑distribution (SED) classes—low‑synchrotron‑peak (LSP), intermediate‑synchrotron‑peak (ISP), and high‑synchrotron‑peak (HSP) blazars—and columns compare the three cadence pairs (7 d vs 3 d, 30 d vs 3 d, 30 d vs 7 d). Table 1 lists the number of sources that survive the quality cuts for each comparison, highlighting the dominance of LSP sources (e.g., 181 LSPs in the 30 d vs 7 d panel).

A clear dichotomy emerges between flat‑spectrum radio quasars (FSRQs), which are predominantly LSPs, and BL Lac objects. FSRQs exhibit the highest σ²_NXS values and the broadest distribution, consistent with γ‑ray emission dominated by external‑Compton (EC) processes that are intrinsically more variable. BL Lacs, especially HSPs, show lower variability amplitudes, reflecting synchrotron‑self‑Compton (SSC) dominated emission and more stable jet conditions. ISP sources appear to occupy an intermediate regime, possibly representing a transition between persistent and episodic variability.

The authors argue that short‑timescale, high‑amplitude variability is a powerful indicator of compact emission zones and efficient particle acceleration, making the identified most luminous and variable AGN prime targets for next‑generation very‑high‑energy (VHE) facilities such as the Cherenkov Telescope Array Observatory (CTAO), the ASTRI Mini‑Array, and the Southern Wide‑field Gamma‑ray Observatory (SWGO). By providing a unified variability metric across multiple timescales, the catalog can be directly incorporated into CTAO population forecasts and observation planning tools. Ongoing work includes extending the analysis to yearly timescales, integrating Swift X‑ray monitoring for multi‑wavelength variability studies, and refining variability‑informed extrapolations for VHE detectability predictions.

In summary, this study fills a gap left by existing Fermi‑LAT catalogs that only report long‑term variability, delivering a high‑cadence, statistically robust variability catalog that enhances our understanding of AGN jet physics and supports optimized observing strategies for upcoming γ‑ray observatories.