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

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 goodquality X-ray data and dynamically measured mass. For low-accreting AGNs (LX/LEdd 6 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 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.