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Astronomers from the P.K. Sternberg State Astronomical Institute (GAISH) at Moscow State University have discovered a key X-ray distinction between Seyfert type I and type II galaxies, as well as galaxies that exhibit rapid type changes. Using X-ray data, the researchers found that the critical factor differentiating these types is the ratio of X-ray luminosity to Eddington luminosity. Specifically, this ratio is high for type I galaxies, low for type II galaxies, and lies at an intermediate value for type-changing galaxies, such as NGC 1566. The findings were published in the prestigious journal Astronomy and Astrophysics.
Seyfert galaxies, a class of active galactic nuclei (AGN), are traditionally classified into two types based on the width of the emission lines in their optical spectra. In type I galaxies, allowed spectral lines are broad, corresponding to Doppler velocities of several thousand km/s, while forbidden lines are narrower, indicating velocities of several hundred km/s. In contrast, both allowed and forbidden lines in type II galaxies are narrow, corresponding to velocities of up to a thousand km/s.
According to the "unified" AGN model, type I and type II Seyfert galaxies are fundamentally the same objects, differing only in their orientation relative to the Earth. Type I galaxies are viewed from above the galactic disk (pole-on), while type II galaxies are seen from the side (edge-on).
Recent discoveries, including galaxies like NGC 1566 that have undergone several type changes, challenge the unified model. These transitions occur over a few months, which cannot be explained by a change in the viewing angle alone. To date, about a hundred such type-changing AGNs are known.
The new study shows that the classification of Seyfert galaxies as type I or type II—and transitions between these types—depend on the X-ray luminosity normalized to the Eddington luminosity. Type I galaxies exhibit a high normalized luminosity, type II galaxies show a low value, and type-changing AGNs have values at the boundary between type I and II.
For example, the transition from type I to type II in NGC 1566 is linked to a sudden drop in X-ray luminosity, caused by a decrease in the accretion rate and a reduction in the proportion of Comptonization of X-ray radiation.
The researchers also estimated the black hole mass in NGC 1566 using X-ray scaling methods. The “X-ray” mass estimate was found to be a hundred times smaller than the “optical” mass estimate, suggesting a potential duality in NGC 1566’s core. This could mean that the variability in luminosity is due to the presence of a second, less massive black hole with a mass of about 2×105 solar masses.
According to Elena Seifina, a leading researcher at GAISH, “Our results strongly suggest that the diversity of Seyfert galaxy types can be explained by the difference in only one parameter—the ratio of the X-ray luminosity to the Eddington luminosity—without requiring additional AGN parameters, such as spatial orientation. This finding blurs the line between type I and type II AGNs, as well as type-changing galaxies.”
This study opens new avenues for X-ray astronomy, offering deeper insights into the radiation mechanisms of massive cosmic objects. The penetrating power of X-ray quanta enables astronomers to explore the inner workings of AGNs more effectively than optical observations alone.
