High-density disc reflection spectroscopy of low-mass active galactic
  nuclei
  The standard alpha-disc model predicts an anti-correlation between the
density of the inner accretion disc and the black hole mass times square of the
accretion rate, as seen in higher mass ($M_{\rm BH}>10^{6} M_{\odot}$) active
galactic nuclei (AGNs). In this work, we test the predictions of the alpha-disc
model and study the properties of the inner accretion flow for the low-mass end
($M_{\rm BH}\approx 10^{5-6}M_{\odot}$) of AGNs. We utilize a new high-density
disc reflection model where the density parameter varies from $n_{\rm
e}=10^{15}$ to $10^{20}$ cm$^{-3}$ and apply it to the broadband X-ray (0.3-10
keV) spectra of the low-mass AGN sample. The sources span a wide range of
Eddington fractions and are consistent with being sub-Eddington or
near-Eddington. The X-ray spectra reveal a soft X-ray excess below $\sim 1.5$
keV which is well modeled by high-density reflection from an ionized accretion
disc of density $n_{\rm e}\sim 10^{18}$ cm$^{-3}$ on average. 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 AGNs. We show that the disc density higher than $10^{15}$ 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.
