Probing the Inner Regions of Changing-Look AGN: NGC 4151

Changing-look active galactic nuclei (CL-AGNs) represent a rare class of AGNs that exhibit significant spectral variability occurring over short timescales of months to decades, and pose challenges to the current AGN unification framework. NGC 4151 is one of the rare types of CL-AGNs which exhibits both transitions between different optical spectral states, as well as transitions between Compton thick and Compton thin AGN. Though there were attempts to constrain the nature of the accretion disk, BLR and torus of this source, most of these studies utilised limited wave band observations (either UV/X-ray or in IR band). Earlier efforts to constrain the properties of torus were mostly limited to IR observation. NGC 4151 is also a rare radio-quiet (RQ) AGN that is detected in γ-rays by Fermi-LAT. However, the physical process responsible for such high energy emissions is still debatable. CL-AGNs are primarily observed in RQ AGNs. However, recent studies show that more than two dozen Fermi blazars exhibit possible transition between BL Lacs and FSRQs, and, hence, are potential radio-loud (RL) CL-AGNs. Observed similarities/differences between RQ and RL CL-AGNs will be crucial to better understand the underlying physical processes responsible for such changes. We carried out one of the first comprehensive broadband spectral energy distribution (SED) studies at different activity states utilising observations from IR to X-ray band. We observed that a two-component torus model along with a conical shell better explains the observed SED. Further, the innermost torus component maximally influences the X-ray, UV and optical emission, whereas the outermost torus and the conical shell have a higher influence on the observed IR emission. We also investigate the γ-ray emission from this source utilising decade-long observation from Fermi-LAT to find any possible connection with the RQ CL-AGNs. The findings of our studies will be presented here.