Dr. Saikruba Krishnan

Active Galactic Nuclei (AGN) are powered by accretion of gas onto supermassive black holes. There are open questions regarding how the X-ray corona, disk, and BLR evolve in response to changes in the global accretion supply. We are using eROSITA's all-sky X-ray surveys to identify extragalactic X-ray transients. I present multi-wavelength observations of a candidate AGN transient event detected with eROSITA. Its X-ray flux increased by $\sim 6$ over six months; concurrent optical photometric monitoring data with ATLAS showed a simultaneous increase. We triggered a multi-wavelength follow-up monitoring program (XMM-Newton, NICER; optical spectroscopy) to study the evolution of the accretion disk, broad-line region, and X-ray corona. During the campaign, X-ray and optical continuum flux subsided over ~ six months. We witnessed a likely sudden strong increase in local accretion rate, which manifested itself via an increase in accretion disk emission and thermal Comptonization emission in the soft X-rays, followed by a decrease in accretion and Comptonized luminosity. The physical processes (e.g., disk instabilities) leading to such substantial variations are still an open question, and future continuous monitoring along with multi-wavelength studies will shed some light on it.