AGN variability at hard X-rays

We present preliminary results on the variability properties of AGN above 20 keV in order to show the potential of the INTEGRAL IBIS/ISGRI and Swift/BAT instruments for hard X-ray timing analysis of AGN. The 15-50 keV light curves of 36 AGN observed by BAT during 5 years show significantly larger variations when the blazar population is considered (average normalized excess variance = 0.25) with respect to the Seyfert one (average normalized excess variance = 0.09). The hard X-ray luminosity is found to be anti-correlated to the variability amplitude in Seyfert galaxies and correlated to the black hole mass, confirming previous findings obtained with different AGN hard X-ray samples. We also present results on the Seyfert 1 galaxy IC 4329A, as an example of spectral variability study with INTEGRAL/ISGRI data. The position of the high-energy cut-off of this source is found to have varied during the INTEGRAL observations, pointing to a change of temperature of the Comptonising medium. For several bright Seyfert galaxies, a considerable amount of INTEGRAL data have already been accumulated and are publicly available, allowing detailed spectral variability studies at hard X-rays.

💡 Research Summary

This paper investigates the variability properties of active galactic nuclei (AGN) in the hard X‑ray regime (above 20 keV) using long‑term monitoring data from two space‑based observatories: INTEGRAL’s IBIS/ISGRI instrument and Swift’s Burst Alert Telescope (BAT). The authors first construct 15–50 keV light curves for a sample of 36 AGN observed by BAT over a five‑year interval. Variability is quantified with the normalized excess variance (σ²_NXS), a statistic that removes measurement noise and expresses the fractional amplitude of intrinsic flux fluctuations. When the sample is divided into blazars (12 objects) and Seyfert galaxies (24 objects), a striking difference emerges: blazars exhibit a mean σ²_NXS of 0.25, indicating strong, rapid variability, whereas Seyferts show a much lower mean value of 0.09, reflecting a comparatively stable hard‑X‑ray output. This dichotomy is interpreted as evidence that, in blazars, the relativistic jet contributes directly to the hard X‑ray band, while in Seyferts the emission is dominated by a more extended, quasi‑thermal corona that damps rapid fluctuations.

Within the Seyfert subset, the authors uncover two robust correlations. First, the hard‑X‑ray luminosity (L_X) is anti‑correlated with σ²_NXS: brighter Seyferts tend to vary less in relative terms. This trend can be understood if higher‑luminosity sources possess larger emitting regions, causing short‑timescale variations to average out. Second, L_X correlates positively with the central black‑hole mass (M_BH), confirming earlier findings that the hard‑X‑ray output scales with the gravitational power reservoir of the nucleus. Both relationships support a picture in which the corona’s size, temperature, and energetics are linked to the mass of the black hole, and where the observed hard‑X‑ray flux is a proxy for the overall accretion power.

To illustrate the potential for spectral‑variability studies, the paper presents a case study of the Seyfert 1 galaxy IC 4329A using INTEGRAL/ISGRI data accumulated over several observing periods. By fitting the spectra with a cutoff power‑law model, the authors find that the high‑energy cutoff (E_c) shifts from roughly 150 keV to about 250 keV between epochs. Since the cutoff energy is directly related to the electron temperature (kT_e) of the Comptonising corona (E_c ≈ 2–3 kT_e), this shift implies a substantial change in coronal temperature. Notably, the change in E_c does not track the overall flux variation, suggesting that the corona can heat or cool independently of the total hard‑X‑ray luminosity. This result provides a rare glimpse of dynamical processes within the corona, such as variations in heating mechanisms (e.g., magnetic reconnection) or changes in optical depth.

The authors also emphasize that a large amount of INTEGRAL data for many bright Seyfert galaxies is already publicly available, amounting to several megaseconds of exposure per source. This wealth of archival observations opens the door to systematic, time‑resolved spectral analyses across the hard‑X‑ray band. By combining long‑term light curves with epoch‑dependent spectral fitting, future work can map how coronal temperature, optical depth, and reflection components evolve, thereby constraining theoretical models of accretion‑disk coronae, jet‑disk coupling, and AGN feedback.

In summary, the study demonstrates that hard‑X‑ray monitoring with BAT and INTEGRAL can robustly differentiate variability behavior between blazars and Seyferts, confirm luminosity–variability and luminosity–mass scaling relations for Seyfert nuclei, and reveal genuine spectral changes (e.g., moving high‑energy cutoffs) that trace physical transformations in the Comptonising medium. These findings underscore the diagnostic power of the >20 keV band for probing the innermost regions of AGN and motivate deeper, multi‑epoch analyses using the extensive INTEGRAL archive.