Understanding the origin of relativistic reflection in AGN using Covariance Spectra

In low-mass, bright, variable AGN, X-ray reverberation has been proven to be a crucial technique for understanding the inner accretion geometry around black holes, but what drives the soft and hard X-ray flux variations across a range of X-ray luminosities is still elusive. In particular, mean spectral modelling provided the idea of the existence of various spectral components in emission geometry. However, no attempts have been made to determine which one is variable in a different timescale. In this work, we have taken an unconventional approach to modelling the Fourier frequency-dependent-ray covariance spectra to answer this question. We have extracted and modeled X-ray covariance and rms spectra of three highly variable low-mass Seyfert galaxies, MCG 6-30-15, NGC 4593 & 1H0707-495, using long-look XMM-Newton/EPIC-pn observation for different Fourier frequencies. Motivated by the fact that the X-ray flux variation is either caused by the variation in absorption, direct non-thermal emission, or reprocessed emission-like reflection, we have used a physical model to fit covariance spectra in different timescales. We found that a variable reflections component is essential to fit the 0.3-9 keV covariance spectra of all three sources: MCG-6-30-15: At higher frequency (25-40 × 10−6 Hz), spectra are dominated by the variability of reflection component with inner accretion disk radius between 1-5 RISCO while at a lower frequency (7.75-11 × 10−6 Hz), the variability is dominated by the direct non-thermal component while the inner disk radius receded to a distance of 5.7-25.8 RISCO, where RISCO is the radius of the innermost stable circular orbit. However, no change in the complex absorption is observed. Similar results are obtained for NGC 4593 and 1H 0707-495. Our analysis suggests that the power-law component dominates low/mid-frequency variability while the reflection component dominates high-frequency, high-flux variability.