Constraining black hole masses in low-accreting AGN using X-ray spectra

TL;DR

This paper explores the limitations of an X-ray scaling method for estimating black hole masses in low-accreting AGNs, proposing an alternative inverse correlation approach that provides reasonable mass constraints from spectral data.

Contribution

It extends the application of X-ray spectral methods to low-accreting AGNs by identifying a new inverse correlation useful for mass estimation.

Findings

The X-ray scaling method is unreliable for low-accreting AGNs.

An inverse Gamma-Lx/LEdd correlation can estimate black hole mass within a factor of 10.

A simple recipe for mass estimation from X-ray spectra is provided.

Abstract

In a recent work we demonstrated that a novel X-ray scaling method, originally introduced for Galactic black holes (GBHs), can be reliably extended to estimate the mass of supermassive black holes accreting at a moderate to high level. Here we investigate the limits of applicability of this method to low-accreting active galactic nuclei, using a control sample with good-quality X-ray data and dynamically measured mass. For low-accreting AGNs, because the basic assumption that the photon index positively correlates with the accretion rate no longer holds the X-ray scaling method cannot be used. Nevertheless, the inverse correlation in the Gamma-Lx/LEdd diagram, found in several low-accreting black holes and confirmed by this sample, can be used to constrain black hole mass within a factor of ~10 from the dynamically determined values. We provide a simple recipe to determine black hole mass using solely X-ray spectral data, which can be used as a sanity check for black hole mass determination based on indirect optical methods.