Covariance Spectral Modeling in AGN

Understanding disc-corona geometry through Xray reverberation studies in Seyfert galaxies is crucial, yet our knowledge mostly relies on flux-averaged mean spectral analysis. In this study, we investigate the origin of the large Xray variability of the Seyfert galaxy NGC 4593 using two XMM-Newton observations, which are at least 65ksec long and have a 0.3 to 10 keV Xray flux difference by a factor of approx 2.5. We extracted mean spectra, Fourier frequency resolved covariance, and time lag spectra, and performed modelling of all spectra in a self-consistent manner. From the best fit covariance spectra for the first time, we have shown that energy-dependent covariance during low flux shows dominance of direct powerlaw continuum over reflection continuum at all fourier frequencies (2.1 to 390 times 10 to the power -5 Hz). However, during high flux, the variabilities are dominated by the reflection components most of the time. Our results are further supported by the Fourier frequency-dependent time lag spectral modelling during high and low fluxes. A significant change is observed in the xray reverberation delay time scale from 483 with uncertainty 135 sec during high flux to less than 96 sec during low flux, indicating a change in coronal size at least by a factor of approx 2 from less than 3.3 Rg to greater than 7.2 Rg during low to high flux transitions.