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Active galactic nuclei (AGNs) are powered by accretion onto supermassive black holes, and they persistently accrete mass from the galactic environment over a long timescale ranging from 10^5-10^6 years. During its lifetime, an AGN typically exhibits stochastic variability from a few percent in the optical band up to a factor of a few in the X-ray band. However, some AGNs occasionally exhibit flux changes above the typical stochastic variability where the variability factor can be greater than an order of magnitude (>~10). This extreme variability is often accompanied by an optical spectral type change, e.g., from type 1 to type 2 and vice versa. These transient sources are known as changing look AGNs (CLAGN). In most cases, the detection of such extremely variable CLAGN AGN transients has been serendipitous. But with the advent of eROSITA, the search for extreme transients has become more systematic. The eROSITA X-ray telescope aboard the Spectrum-Roentgen/Gamma has conducted all-sky scans every six months.
Using these scans, we detect extreme extragalactic AGN transients as they are happening. Subsequently, we trigger multiple follow-up X-ray observations using XMM-Newton, Swift, and optical spectroscopic and photometric follow-up observations to carry out extensive long-term monitoring. Here, I present a few candidate ignition and shutdown events detected with eROSITA and followed up by our multiwavelength observation campaigns. Furthermore, I also present an archival observational study of the long-term CLAGN source Mrk 1018. In these sources, we track the changes in emission from various AGN substructures like the accretion disk, corona, broad line region, and torus in real-time as they respond to significant changes in the accretion flow. Our research thus helps discern the various modes of accretion onto supermassive black holes, the properties of the broad line emitter, and in general, the morphology of the circumnuclear gas. |