Chandra Survey of Nearby Galaxies: Testing the Accretion Model for Low-luminosity AGNs

TL;DR

This study uses Chandra observations to examine low-luminosity AGNs, finding that their X-ray absorption properties correlate with accretion rates, supporting hot accretion flow models and outflow-related absorption.

Contribution

It provides observational evidence linking X-ray absorption and outflows in low-luminosity AGNs, supporting the hot accretion flow model over other scenarios.

Findings

Absorption column density scales with Eddington ratio in LLAGNs.

X-ray spectra are softer at low luminosities.

Outflows are likely responsible for X-ray absorption.

Abstract

From a Chandra sample of active galactic nuclei (AGNs) in nearby galaxies, we find that for low-luminosity AGNs (LLAGNs), either the intrinsic absorption column density, or the fraction of absorbed AGNs, positively scales with the Eddington ratio for $L_{\rm bol}/L_{\rm Edd} \lesssim 10^{-2}$. Such a behavior, along with the softness of the X-ray spectrum at low luminosities, is in good agreement with the picture that they are powered by hot accretion flows surrounding supermassive black holes. Numerical simulations find that outflows are inevitable with hot accretion flows, and the outflow rate is correlated with the innermost accretion rate in the low-luminosity regime. This agrees well with our results, suggesting that the X-ray absorption originates from or is associated with the outflow material. Gas and dust on larger scales may also produce the observed correlation. Future correlation analysis may help differentiate the two scenarios.