Constraining black hole masses in low-accreting active galactic nuclei using X-ray spectra

Abstract

In a recent work we demonstrated that a novel X-ray scaling method, originally introduced for Galactic black holes (BHs), can be reliably extended to estimate the mass of supermassive BHs accreting at a moderate to high level. Here we investigate the limits of applicability of this method to low-accreting active galactic nuclei (AGN), using a control sample with goodquality X-ray data and dynamically measured mass. For low-accreting AGN(LX/LEdd ≤10-4), 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 G{cyrillic}-LX/LEdd diagram, found in several low-accreting BHs and confirmed by this sample, can be used to constrain MBH within a factor of ~10 from the dynamically determined values. We provide a simple recipe to determine MBH using solely X-ray spectral data, which can be used as a sanity check for MBH determination based on indirect optical methods. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.