A Multiepoch X-Ray Study of the Nearby Seyfert 2 Galaxy NGC 7479: Linking Column Density Variability to the Torus Geometry

Active galactic nuclei (AGN) are powered by accreting supermassive black holes, surrounded by a torus of obscuring material. Recent studies have shown how the torus structure, formerly thought to be homogeneous, appears to be ‘patchy’: the detection of variability in the line-of-sight (los) hydrogen column density (N_H,los), in fact, matches the description of an obscurer with a more complex structure made of clouds with different density. In this work we perform a multi-epoch analysis of the Seyfert 2 galaxy NGC 7479’s X-ray spectra, covering a time span of ~20 years. This allowed us to put tight constraints on its torus global properties such the covering factor, the inclination angle and the average column density. The N_{H_los} variability we measured between different epochs strongly favors a clumpy torus scenario. The best fit parameters suggests a disentanglement of the reflection and the absorption material inside the torus; in particular the presence of a thick (>10²⁵ cm^-2) “inner ring” is required to explain the reflection dominated spectra. The ongoing ALMA campaign focused on the direct imaging of this inner structure will also be discussed.