Most of the Cosmic X-ray Background (CXB) radiation from 1-100 keV is the result of AGN emission. The contribution from the unobscured AGN population, responsible for the CXB emission at <10 keV, has been almost completely resolved. However, the detection of obscured AGN, which are responsible for a significant fraction (~40% at peak) of the CXB, is found to be challenging. By observation, the obscured Compton-Thick (CT: column density >10^24 /cm^2 ) AGN fraction in the local universe is (10-50)% lower than what expected from CXB population synthesis models. Therefore, an almost complete census of obscured AGN is needed at different wavelengths to fill the gap between observations and model predictions. It is commonly accepted that the nuclear obscuration in an AGN-galaxy (at least at low redshift) is caused by the circum-nuclear material of molecular and dusty clouds called "torus", but its geometrical, physical, and chemical properties are far from being accurately known. On that path, I will present a comprehensive and systematic analysis of a sample of few local obscured AGN candidates (z<0.05) through X-ray spectral analysis using Chandra and XMM-Newton at E<10 keV, coupled with NuSTAR data at E>10 keV, to characterize the torus obscuration. The purpose is to update the census of CT-AGN by studying the different properties of the torus over the energy range 0.3-50 keV using physically motivated torus models like- MyTorus, borus02, UxClumpy, Xclumpy. Adding our analysis to the previous results, we find the total population of local CT-AGN almost reduces to half when using NuSTAR observation. We also find around 78% of the population shows a clumpy nature of torus. In my talk, I will also briefly mention a self-consistent multi-wavelength analysis, i.e. a joint analysis combining the mid-IR SED-derived view of the obscuring medium with that from X-rays, using CIGALE. In addition to this torus-focused study, I will highlight our ongoing investigation into the extreme and dynamic behavior of the changing-look AGN 1ES 1927+654.
Dr. Dhrubojyoti Sengupta is a postdoctoral researcher currently affiliated with the University of Maryland, College Park, in collaboration with SURA and NASA Goddard Space Flight Center. He completed his Ph.D. in Astrophysics at the University of Bologna (in collaboration with INAF–OAS Bologna) last year, where his research focused on obscured active galactic nuclei (AGN) in the local Universe, using multi-epoch X-ray data and mid-infrared SED modeling. His work combines multi-wavelength studies—particularly in the X-ray, infrared, and optical/UV bands—to understand AGN variability, torus structure, and changing-look phenomena.