The low-mass end of accreting supermassive black holes*

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 n_e =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 n_e ~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.