Presentation #230.08 in the session AGN and Quasars IV.
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 (MBH ~ 105-106 MΘ) 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 develop a new high-density disc reflection model and apply it to the broadband X-ray 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. 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 higher disc density 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.