Black hole mass estimation from X-ray variability measurements in active galactic nuclei

We propose a new method of estimation of the black hole masses in active galactic nuclei (AGN) based on the normalized excess variance, σ²_nxs. We derive a relation between σ²_nxs, the length of the observation, T, the light-curve bin size, Δt, and the black hole mass, assuming that (i) the power spectrum above the high-frequency break, ν_bf, has a slope of −2, (ii) the high-frequency break scales with black hole mass, (iii) the power-spectrum amplitude (in frequency–power space) is universal and (iv) σ²_nxs is calculated from observations of length T < 1/ν_bf. Values of black hole masses in AGN obtained with this method are consistent with estimates based on other techniques such as reverberation mapping or the M_BH–stellar velocity dispersion relation. The method is formally equivalent to methods based on power spectrum scaling with mass, but the use of σ²_nxs has the big advantage of being applicable to relatively low-quality data.