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

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 ($M_{\mathrm{BH}}>10^6{M}_{\odot }$) 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 ($M_{\mathrm{BH}}\approx 10^{5-6}{M}_{\odot }$) of AGNs. We utilize a new high-density disc reflection model where the density parameter varies from $n_{\mathrm{e}}=10^{15}$ 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 $n_{\mathrm{e}}\sim 10^{18}$ 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.

arXiv comment20 pages, 10 figures, 6 tables. Accepted for publication in MNRAS

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