Speaker
Description
Active Galactic Nuclei (AGN) are powered by accretion of gas onto supermassive black holes. Persistently accreting AGNs, alongside inherent stochastic variability, sometimes display extreme flux and spectral changes (X-ray and/or optical fluxes), usually linked to significant variations in the global accretion rate. These drastic changes in the continuum flux are sometimes accompanied by significant changes in the optical spectra, specifically the fluxes of broad components to Balmer emission lines. There are still 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 such extragalactic X-ray transient AGNs. This presentation delves into these aspects and discusses an intriguing X-ray flare observed in a Seyfert galaxy detected by eROSITA. Its X-ray flux increased by ∼ 5 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). 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.
