On the relationship between black hole mass and X-ray variability amplitude in the low-mass regime of active galactic nuclei

Recent studies of active galactic nuclei (AGN) found a statistical inverse scaling between the X-ray normalized excess variance σ_rms ² (variability amplitude) and the black hole mass spanning over M _BH = 10⁶ - 10⁹ M _⊙. We present a study of this relation by including AGN with M _BH = 10⁵ - 10⁶ M _⊙. It is found that the relation is no longer a simple extrapolation of the known inverse proportion, but starts to flatten around 10⁶ M _⊙. This behavior can be understood by the shape of the power spectrum density of AGN and its dependence on the black hole mass.