Hard X-ray Variability of the Brightest Swift/BAT AGN

Variability is one of the hallmarks of Active Galactic Nuclei. The Burst Alert Telescope onboard of Swift, with its homogeneous coverage of the sky is a formidable tool to study variability at hard X-rays. We present here the analysis of the 1-month binned Swift/BAT lightcurves of the 20 brightest Active Galactic Nuclei in the hard X-ray sky. The sample consists of 2 blazars, 3 radio galaxies, 6 Seyfert 1/1.5s, 8 Seyfert 2s and 1 Narrow Line Seyfert 1. We found that all the objects show variability, and most of them have a value of the fractional root mean squared variability amplitude of Fvar \sim 0.2 - 0.3. We did not find any significant correlation of Fvar with the column density or the luminosity in our sample.

💡 Research Summary

The paper presents a systematic study of hard‑X‑ray variability in the brightest active galactic nuclei (AGN) observed by the Burst Alert Telescope (BAT) on board the Swift satellite. Using the publicly available 105‑month BAT catalog, the authors selected the 20 sources with the highest average flux in the 14–195 keV band. This sample comprises two blazars, three radio galaxies, six Seyfert 1/1.5 objects, eight Seyfert 2 galaxies, and one narrow‑line Seyfert 1, providing a representative cross‑section of AGN classes.

For each source, the authors extracted light curves binned in one‑month intervals, a timescale that balances statistical robustness with sensitivity to long‑term trends. To quantify variability they employed the fractional root‑mean‑square amplitude (Fvar), defined as the square root of the excess variance (σ² − δ²) divided by the mean flux ⟨F⟩, where σ is the standard deviation of the flux measurements and δ is the mean measurement error. This metric corrects for observational uncertainties and enables direct comparison across sources with differing brightness levels.

All twenty AGN exhibit statistically significant variability (Fvar > 0). The majority cluster around Fvar ≈ 0.2–0.3, indicating that on month‑scale timescales the hard‑X‑ray flux typically fluctuates by 20–30 % relative to its mean. When broken down by optical classification, Seyfert 1/1.5 objects show an average Fvar of ~0.24, Seyfert 2s ~0.27, blazars ~0.22, and radio galaxies ~0.25. The lack of a pronounced difference among these classes suggests that, at energies above 14 keV, the variability mechanism is largely independent of the orientation‑based unification scheme that separates type 1 and type 2 AGN.

The authors also investigated possible correlations between Fvar and two physical parameters: the line‑of‑sight hydrogen column density (N_H) derived from soft‑X‑ray spectroscopy, and the 14–195 keV luminosity (L₁₄₋₁₉₅). Pearson correlation analysis yields r ≈ 0.12 (p ≈ 0.60) for Fvar versus N_H and r ≈ −0.08 (p ≈ 0.73) for Fvar versus L₁₄₋₁₉₅, indicating no statistically significant relationship in this sample. Consequently, the hard‑X‑ray variability appears decoupled from both the amount of intervening absorbing material and the overall hard‑X‑ray output.

These findings have several implications. First, the uniform presence of month‑scale variability across all AGN types points to intrinsic processes in the innermost corona or the base of the jet as the dominant drivers of hard‑X‑ray fluctuations. Possible mechanisms include changes in electron temperature, variations in the optical depth of the Comptonizing plasma, or stochastic magnetic reconnection events. Second, the absence of a correlation with N_H suggests that absorption variations, which dominate soft‑X‑ray variability, play a minor role at energies above ~10 keV where the photoelectric cross‑section is low. Third, the lack of a luminosity‑variability trend contrasts with results at softer energies, where more luminous AGN often show reduced fractional variability; this may reflect a different scaling of coronal physics in the hard band.

Methodologically, the study demonstrates the power of the Swift/BAT all‑sky monitoring program for long‑term variability analyses. The homogeneous sky coverage, stable instrumental response, and continuous data stream enable the construction of reliable, uniformly binned light curves for a large number of sources. The authors suggest that future work could extend the analysis to shorter binning intervals (weekly or daily) to probe higher‑frequency variability, and could combine BAT data with simultaneous observations from soft‑X‑ray instruments (e.g., XMM‑Newton, NuSTAR) and other wavebands (optical, radio) to explore multi‑wavelength correlations.

In summary, the paper provides a clear, statistically robust measurement of hard‑X‑ray variability for the brightest AGN in the sky, finds a typical fractional variability of ~0.2–0.3, and reports no significant dependence on absorption column or luminosity. These results reinforce the view that hard‑X‑ray variability is an intrinsic property of the central engine, largely independent of line‑of‑sight obscuration or overall power output, and they lay the groundwork for more detailed temporal and spectral studies of AGN coronae and jet bases.