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The unification theory of black hole accretion suggests that the accretion process is independent of the black hole mass, and we expect to observe similar accretion properties in sources spanning a wide range of black hole mass. The discovery of scaling relations between the black hole mass and X-ray reverberation lag for both accreting supermassive and stellar-mass black holes is a breakthrough in this context. However, the low-mass end of active galactic nuclei (AGN) has never been explored in detail. For this purpose, we construct a sample of the least-massive AGN from the XMM-Newton archive and measure frequency-resolved time delays between the soft and hard X-ray emission as well as test the predictions of the standard alpha-disc model. We utilize a new high-density disc reflection model where the density parameter varies from ne=1015 to 1020 cm-3 and apply it to the broadband X-ray (0.3-10 keV) spectra of the sample. The X-ray spectra reveal soft X-ray excess below around 1.5 keV, which is well modeled by high-density reflection from an ionized accretion disc of density ne ~1018 cm-3 on average. We detected soft reverberation time lags in some of these AGN which provides the strongest supporting evidence for the reflection origin of the soft X-ray excess. The results suggest a radiation pressure-dominated disc with an average of 70% fraction of the disc power transferred to the corona, consistent with that observed in higher mass AGN. We show that the disc density higher than 1015 cm-3 can result from the radiation pressure compression when the disc surface does not hold a strong magnetic pressure gradient. We find tentative evidence for a drop in black hole spin at low-mass regimes, which hints that the merger of intermediate-mass black hole pairs could be responsible for the formation of the low-mass end of supermassive black holes, which can be confirmed with the future gravitational wave detector LISA. &nbsp

Active Galactic Nuclei (AGN) have been of great interest because of their complex broadband electromagnetic spectra and extreme variability ranging from radio to gamma-rays at the time-scales from a few minutes to years depending upon the physical processes they are governed by. In order to understand the complex nature of AGN X-ray variability, I devised a method to model the fractional root-mean-squared (FRMS) variability spectra and measured the variability power in different spectral components. We applied this method to investigate the nature and origin of energy-dependent variability of various Type 1 AGN. In this talk, I will present several studies with the aim of better understanding the variability mechanism and hence the accretion processes onto the supermassive black holes.

The scaling relations between the black hole (BH) mass and soft lag properties for both AGN and BH X-ray binaries (BHXRBs) indicate the same underlying physical mechanism at work in accreting BH systems spanning a broad range of mass. However, the low-mass end of AGN has never been explored. In this work, we extend the existing scaling relations to lower-mass AGN, which serve as anchors between the normal-mass AGN and BHXRBs. For this purpose, we construct a sample of extremely low-mass AGN from the XMM-Newton archive and measure frequency-resolved time delays between the soft and hard X-ray emission. We report that the soft band lags behind the hard band emission at high frequencies, which is interpreted as a sign of reverberation from the inner accretion disc in response to the direct coronal emission. At low frequencies, the hard band lags behind the soft band variations, which we explain in the context of the inward propagation of luminosity fluctuations through the corona. We find that the X-ray source for the sample extends at an average radius of around 6 gravitational radii and a median height of around 10 gravitational radii above the disc plane, consistent with gravitational microlensing observations. Our results confirm that the scaling relations between the BH mass and soft lag amplitude/frequency derived for higher-mass AGN can safely extrapolate to lower-mass AGN, and the accretion process is indeed independent of the BH mass. &nbsp