An X-ray study of changing-look active galactic nuclei

A significant number of changing-look active galactic nuclei (CL AGNs) have been identified to date. In this work, we study what happens to the X-ray spectrum during CL events. We use the example of the nearby CL Seyfert named NGC 1566, which has been observed by Swift, NuSTAR, XMM-Newton , and Suzaku . We applied the Comptonization model to describe the evolution of NGC 1566 X-ray spectra during outbursts and compared these results with the typical behavior of other AGNs to identify some differences and common properties that will ultimately help us better understand the physics of the CL phenomenon. We found that changes in the X-ray properties of NGC 1566 are characterized by a different combination of Sy1 (using 1H 0707–495 as a representative) and Sy2 properties (using NGC 7679 and Mrk 3 as their representatives). At high X-ray luminosities, NGC 1566 exhibits behavior typical of Sy1. At low luminosities, we see a transition of NGC 1566 from Sy1 behavior to an Sy2 pattern. We revealed the saturation of the spectral indices, α , for these four AGNs during outbursts ( α 1566 ∼ 1.1, α 0707 ∼ 2, α 7679 ∼ 0.9, and α mrk 3 ∼ 0.9) and we determined the masses of the black holes (BHs) in the centers of these AGNs; namely, M 0707 ∼ 6.8 × 10 7 M ⊙ , M 7679 ∼ 8.4 × 10 6 M ⊙ , M mrk 3 ∼ 2.2 × 10 8 M ⊙ , and M 1566 ∼ 2 × 10 5 M ⊙ , applying the scaling method. Our spectral analysis shows that the changing-look of NGC 1566 from Sy1.2 to Sy1.9 in 2019 was accompanied by the transition of NGC 1566 to an accretion regime, which is typical for the intermediate and highly soft spectral states of other BHs. We also find that when going from Sy2 to Sy1, the spectrum of NGC 1566 shows an increase in the soft excess accompanied by a decrease in the Comptonized fraction (0.1 < f < 0.5), which is consistent with the typical behavior of BH sources during X-ray outburst decay. Our results strongly suggest that the broad variations in behavior observed among CL, Sy1, and Sy2 AGNs with different X-ray luminosities can be explained by changes in a single variable parameter (e.g., the ratio of the AGN’s X-ray luminosity to its Eddington luminosity), without any need for incorporating additional differences in the Sy AGN parameters (e.g., inclination). Therefore, we find that the distinction between the Sy1, Sy2, and CL-AGN subclasses is effectively blurred.