  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.
