| Abstract: | In low-mass, bright, accreting 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 luminosites is still elusive.In this work, we have taken an unconventional approach to modeling the Fourier frequency-dependent X-ray covariance spectra to answer this question.We have extracted X-ray covariance and rms spectra of two highly variable low-mass Seyfert galaxies, MCG 6-30-15 and NGC 4593, using long-look XMM-Newton EPIC-pn observation and they are modeled as a function of photon energy 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 frequency regimes. We found that complex absorption, relativistic reflections, and variable power-law are essential to fit the 0.3-9 keV covariance spectra of 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 R _ISCO while at 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 R_ISCO , where R _ISCO is the radius of the innermost stable circular
orbit. While transiting from a lower to a higher frequency regime, normalization of the variability caused by reflection increases by ∼2.75 times. However, no change in the complex absorption is observed. Similar results are obtained for NGC 4593. Our analysis suggests that low frequency, low flux variability is dominated by the power-law component while high frequency, high flux variability is dominated by the relativistic reflection component. |
