High-density disc reflection spectroscopy of low-mass active galactic nuclei

TL;DR

This study investigates the inner accretion disc properties of low-mass AGNs using high-density reflection models, confirming the alpha-disc model predictions and revealing a soft X-ray excess consistent with high-density, ionized discs.

Contribution

It introduces a new high-density disc reflection model applied to low-mass AGNs, providing insights into disc density, ionization, and the accretion physics at the low-mass end.

Findings

High-density reflection explains the soft X-ray excess in low-mass AGNs.

Disc densities around 10^{18} cm^{-3} are consistent with radiation pressure compression.

Evidence suggests a possible decrease in black hole spin at low masses.

Abstract

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.