2025ApJ...981L..18R

Variability studies offer a compelling glimpse into black hole dynamics, and Neutron Star Interior Composition Explorer's (NICER's) remarkable temporal resolution propels us even further. NICER observations of an active galactic nucleus (AGN), NGC 4051, have charted the geometry of the emission region of the central supermassive black hole. Our investigation of X-ray variability in NGC 4051 has detected extreme variations spanning a factor of 40-50 over a mere 10-12 hr. For the first time, we have constrained the X-ray power spectral density (PSD) of the source to 0.1 Hz, corresponding to a temporal frequency of 10⁴ Hz in a galactic X-ray binary with a mass of 10 M_⊙. No extra high-frequency break/bend or any quasiperiodic oscillations are found. Through detailed analysis of energy-dependent PSDs, we found that the PSD normalization, the high-frequency PSD slope, as well as the bending frequency remain consistent across all energies within the 0.3-3 keV band, revealing the presence of a constant temperature corona. These significant findings impose critical constraints on current models of X-ray emission and variability in AGN.